Known issues: 2026.x releases

Note: This page lists known issues valid for firmware releases 2026.0 LTS up to the most recent. 

A firmware release mentioned in this context encompasses all hotfixes for the respective release. For example, if 2024.0 LTS is mentioned, this means actually 2024.0.x LTS. If a hotfix for a major release fixes an issue then we're writing that explicitly down.

Security shieldFor security issues and possible vulnerabilities, see the security advisories of the Product Security Incident Response Team (PSIRT) of Phoenix Contact via its website.
In addition, vulnerabilities found with firmware 2026.0 LTS or newer are listed in the Vulnerabilities and exposures section.

Firmware, operating system, and Web-based Management

Reset after changes in Overlay File System

Known from firmware 2021.6 on AXC F 3152:

Reset type 2 can only be performed with a running Linux system. If a user makes changes in the Overlay File System (overlayFS) that lead to problems booting the system, then resetting the device via the reset button is not possible.  
Only known workaround: Boot the controller with an external SD card, copy the overlayFS to this external SD card and delete the overlayFS on the controller after that.

Downgrading after debugging causes kernel panic

Known from firmware 2025.0 on all supported PLCnext Control devices except Virtual PLCnext Control:

If downgrading from firmware ≥2025.0 to firmware ≤ 2024.6 while the DEBUG logging is active or has been activated previously (i.e. the file /etc/plcnext/device.log.ini exists in the Overlay File System) then a kernel panic may occur. This situation can only be resolved by resetting the PLC to factory defaults type 1 (see Reset).

Higher RAM and CPU utilization from firmware 2025.0

Known from firmware 2025.0 on all supported PLCnext Control devices:

Before updating from firmware ≤2024.6 to newer releases, be aware that those firmware releases contain some fundamentally updated parts of the underlying Linux® operating system and adapt the PLCnext Runtime System to these changes, which results in higher RAM and CPU utilization. This in turn can have an impact on existing applications if these applications are close to the maximum RAM and CPU performance of the respective PLC type. For such applications, Phoenix Contact recommends also checking the RAM and CPU utilization when validating the functionality after a firmware and application update.

WBM 2 problem with Chromium-based browsers

Known from firmware 2025.0 up to 2026.0.4 LTS on all supported PLCnext Control devices – fixed with 2026.0.5 LTS:

After updating a Chromium-based web browser on the PC, the drop-down lists in the WBM can no longer be operated. This has been observed with Microsoft Edge from version 149.0.4022.52 as well as with Google Chrome from version 149.0.7827.54 on devices with the Web‑based Management 2, i.e. firmware releases from 2025.0 up to 2026.0.4 LTS. Since Mozilla's Firefox browser is not affected, the Firefox browser can be used as workaround. This issue is fixed in the firmware version 2026.0.5 LTS.

Hardware settings and indicators

"Link" and "Active" LED not working as expected

Known for firmware 2021.0 LTS or newer on AXC F 1152 and AXC F 2152:

The Link and the Active LEDs on the X1 and X2 network interfaces are not illuminated when a 10BaseT connection is used.

Network

DHCP

Known for firmware 2020.6 and newer on all supported PLCnext Control devices:

DHCP can only be activated for Ethernet adapters that are not assigned as PROFINET controllers or PROFINET devices. To make the settings effective in the network, the device must be restarted.

Known for firmware 2020.6 on all supported PLCnext Control devices, and persisting from firmware 2021.0 LTS and newer in the touch display of RFC 4072S:

In general, when DHCP is switched on, the current IP settings are not yet displayed in the WBM or on the touch display, but the static settings last set are displayed.

PROFINET

PROFINET cycle time

Known from firmware 2021.0 LTS on AXC F 1152 and AXC F 2152:

The use of a PROFINET cycle time of 1 ms leads to a deviation of the jitter behavior required by the controller certification. Operation in  this state is possible but not recommended.

PROFINET PRL not supported

Known from firmware 2021.6 on AXC F 2152, AXC F 3152, and RFC 4072S:

PROFINET PRL (Phoenix Redundancy Layer) is not supported. Firmware release 2021.0 LTS was the last release supporting PRL, newer releases do not support this feature.

PROFINET controller status not updated in WBM tab

Known from firmware release 2023.0 LTS on RFC 4072R:

In the Diagnostics → Profinet WBM page, the Overview tab fails to update the status of the PROFINET controller. The Profinet Controller Function is indicated as Deactivated even if the PROFINET controller is active and operating its connected PROFINET devices. Nevertheless, in the Device List and Tree View tabs the status is indicated correctly. Will be fixed soon.

PROFINET watchdog due to increased CPU load

Known from firmware release 2024.6 on all supported PLCnext Control devices:

The PROFINET controller stack had to be adapted to the increased requirements of the PROFINET certification test, resulting in an increased CPU load in ESM 1. In exceptional cases, this can lead to a task watchdog in PLC projects with many PROFINET connections and tightly set ESM task watchdog times.

Name of the PROFINET controller interface moves to the name of the PROFINET device interface

Known from firmware release 2024.6 on all supported PLCnext Control devices:

If the PROFINET controller is deactivated when a device with more than one network interface is initialized for the first time, the name of the PROFINET controller interface moves to the name of the PROFINET device interface. However, after this shift a new assignment of the correct name by the user is then permanently adopted.

PROFIBUS

Faulty PROFIBUS configuration

Known from firmware 2021.9 on AXC F 2152 and AXC F 3152 with AXC F XT PB attached:

If a faulty PROFIBUS configuration is set after successful first commissioning, this is not displayed via the system variables or the diagnostic interface. The affected PROFIBUS station is not commissioned, but is displayed as ready.

EtherNet/IP™

EtherNet/IP™ ports with active firewall

Known for all firmware releases on AXC F 1152, AXC F 1252, AXC F 2152, and AXC F 3152 devices:

If the firewall is activated via WBM, the operation of EtherNet/IP™ is no longer possible. This can be remedied by subsequently activating the ports:

  • Incoming connections: port 44818
  • Outgoing connections: port 2222

PLCnext Engineer

HMI pages "busy" during program downloads

Known from firmware 2020.0 LTS on all supported PLCnext Control devices:

During a PLCnext Engineer program download (both total and "changes only"), the web server returns a 503 error ("busy") for requests to the HMI pages.

PLC error when breakpoint is executed within some ESM event tasks

Known from firmware 2020.3 on all supported PLCnext Control devices:

Using PLCnext Engineer, setting and activating a breakpoint in a code which is located in one of the ESM event tasks "Cold start", "Warm start", "Hot start" or "Stop" can cause an error: If this event task is executed, then the communication with PLCnext Engineer is interrupted and the PLC switches to an error state. The PLC needs to be rebooted.

PLCnext Engineer project lost after file operations

Known for firmware 2025.0 and 2025.6 on Virtual PLCnext Control:

When attempting to use Download Changes in PLCnext Engineer with unsupported modifications (such as adding an ESM task), the operation is rejected. Unexpectedly, after restarting the container or the firmware, the entire PLCnext Engineer project is lost. This issue will be remedied in an upcoming firmware release. As a workaround for moving files, merge the different volumes in the related docker-compose file (.yml) into a single one (show an exampleshow an example).

Change from
volumes:
    - projects:/opt/plcnext/projects
    - config:/opt/plcnext/config
    - data:/opt/plcnext/data
    - apps:/opt/plcnext/apps
    - logs:/opt/plcnext/logs
to    
volumes:
    - plcnext:/opt/plcnext

PLCnext Engineer Simulation installation is aborted

Known for PLCnext Engineer Simulation 2025.0 and 2026.0 LTS:

When installing the 2025.0 or 2026.0 LTS implementation of the PLCnext Engineer Simulation on a computer that has still an older simulation installed, then the 2025.0 installation process is aborted. Before installing the 2025.0 or 2026.0 LTS implementation you first have to uninstall the older implementations and and later re-install them, if necessary.

PLCnext Store

License operations

Known from firmware 2021.9 on all supported PLCnext Control devices:

Licence operations, such as adding or removing a licence, include cryptographic operations and hence shall only be performed if the PLC is stopped. This may avoid side effects due to preempting the license operations by tasks running with higher priority.

Programming

Task naming

Known from firmware 2020.6 on all supported PLCnext Control devices:

If Event, EventTask, ServiceTask or Globals is used as the name of a task in a user program then an error condition of the device occurs when downloading the project to the device. This is because these class names are already used internally.

Stop task not executed after exception

Known for firmware 2026.0 LTS up to the 2026.0.5 LTS hotfix on all supported PLCnext Control devices – fixed with firmware 2026.6:

When an ESM task causes an exception then the ESM event task "exception" is executed, and after that the ESM event task "stop" is executed. From firmware 2026.0 LTS up to the 2026.0.5 LTS hotfix, erroneously only the "exception" task was executed. This has been corrected in firmware 2026.6.

PLCnext Technology toolchain

PKCS type support

Known for PLCnext Technology Toolchain from 2025.0:

Only PKCS #1 PEM files are supported. This can be achieved with the -traditional flag when generating a private key with openssl, e.g.:
openssl genrsa -aes256 -traditional -out privatekey.pem 2048

Visual Studio® solution builds

Known for PLCnext Technology Toolchain from 2025.0:

Solutions with several PLCnext Technology C#/C++ projects in Visual Studio® cannot be built. Each project must be built separately.

Visual Studio® error "Value does not fall within the expected range."

Known for PLCnext Technology Toolchain from 2024.0:

This issue appears with Visual Studio 2022 version 17.9.2 and newer. A bugfix needs to be found. Meanwhile, use this workaroundthis workaround.

  • Start a project without a target and add a solution configuration manually named Project-specific. After that, it should be possible to add targets to the project and work as usual.
  • Open the configuration manager.
  • Add a new configuration.
  • Name the new configuration Project-specific and uncheck the checkbox Create new project configuration.

Special characters in a Windows® user name

Known for PLCnext Technology Toolchain from 2023.6:

CMake is not able to work with Windows® user names containing special characters and special letters. This is an issue in CMake and cannot be fixed by Phoenix Contact. The solution is to use a user name without special characters and preferably even without whitespace (blanks).

Uninstallation issues with the PLCnext Technology Toolchain

Known for PLCnext Technology Toolchain from 2023.6:

In some cases removing of the PLCnext Technology C++ Templates in Visual Studio® 2022 fails.

  • After uninstalling the toolchain (or if installing a newer version fails), check if the former template is still available in Visual Studio®. In case, uninstall the former template manually in Visual Studio® 2022 → Extensions → Manage Extensions → Installed. 
  • Check if it is still available after uninstalling the rest of the toolchain or a failed installation and remove it manually. 
  • If you have further issues, please contact our support via the PLCnext Community forum.

SDK installer on Linux®

Known for PLCnext Technology Toolchain from 2023.0:

In rare cases using the install sdk command throws the error Did not find the expected usage information.. Please contact our support if this happens.

SDK installation via Visual Studio®

Known for PLCnext Technology Toolchain from 2023.0:

Error windows during installation of a PLCnext SDK via Visual Studio® sometimes do not appear in foreground. This could be misleading and looks like the installation is running endlessly.

Shared Native Libraries

Known for PLCnext Technology Toolchain from 2023.0:

Mixed mode (implementation code in C# and C++) is not fully implemented and will not be supported for now. Further information and documentation will follow. 
In the Linux® toolchain, building C# and Shared Native Libraries is not yet fully available.

Retain data

Retain variables

Known for all firmware releases on all supported PLCnext Control devices:

After a system watchdog, only a cold start can be performed when the controller is started. The retain variables are set to their respective initialization values.

Known for firmware 2020.0 LTS or newer on all supported PLCnext Control devices:

If a firmware 2020.0 LTS or newer is downgraded to 2019.9 or older and then upgraded again to firmware version 2020.0 LTS or newer, a Cold Restart is performed. The retain variables are set to their default values.

Known for firmware 2020.3 or newer on all supported PLCnext Control devices:

If a requested Warm Restart is impossible to execute via PLCnext Engineer, an implicit Cold Restart is automatically executed and the retain variables are then set to their initialization value. From firmware 2021.0 LTS on, a dedicated state of the retain values can be restored from a data file.

Apps and containers

Error during installation of OCI container apps

Known from firmware 2025.0 or newer on Virtual PLCnext Control:

Re-installation of OCI container apps does not work with Virtual PLCnext Controls. An error is written to /var/log/syslog "Error: faccessat /opt/plcnext/apps/mounted/ ... tar.gz: no such file or directory". 
Remedy: After de-installation of the app, first restart the container and thereafter install the app again.

Safety

Non-safety variable is not written to the standard variable

Known for firmware version 2024.0 LTS on AXC F 2152 with left-aligned SPLC 1000 or SPLC 3000:

If a standard variable in PLCnext Engineer is assigned to a safety variable and to a standard variable, then the standard variable is not written. This issue will be fixed a.s.a.p.

RSC services

Long String variables and WString variables

Known for firmware 2020.3 or newer on all supported PLCnext Control devices:

Access to long STRING variables outside the application is limited to 511 bytes. This concerns reading and writing via the RSC services IDataAccessService and ISubscriptionService. These services are used by OPC UA®, PLCnext Engineer HMI and the online functions of PLCnext Engineer, among others.

The same applies for WString variables (available from firmware 2021.6). Please note that WString variables are converted to UTF8 when accessed via RSC services.

OPC UA® Server

Application URI is not supported

Known from firmware 2021.0 LTS on all supported PLCnext Control devices:

Starting with firmware 2021.0 LTS, the application URI is checked against the client side certificate. Some clients do not support the application URI so the OPC UA® Server declines a connection; e.g. Visu+ ≤ v2.54 throwing an -2145976320 error code. This issue only comes up when using a PLCnext Engineer template ≥ 2021.0 LTS.

Remedy: Checking the application URI can be disabled in the OPC UA® Server configuration as shown in this Phoenix Contact Knowledge Base article.

DataLogger

Multiple DataLogger sessions

Known from firmware 2020.0 LTS on all supported PLCnext Control devices:

If two or more DataLogger sessions are configured to write to the same database, only the data of one session will be transferred to the database on the SD card at the end. The user does not receive a message that not all data is saved.

 

 

• Published/reviewed: 2026-07-31  ☀  Revision 099 •