This Beckhoff manual page describes why the PLC would respond with modbus exception code 4
the text is
Watchdog or K-bus error
EK9000: E-bus error
and they also make clear that illegal modbus commands would produce different EC to 4
On that basis, I’d say your openHAB config is fine, and the problem is with your PLC config.
It seems odd to both appear to “work” and report an error, but I suppose sometimes the PLC has to try to implement a command to discover an error, so there is a sort of sense there.