Modbus RTU FC05 gives Error Code 84


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PLC: S7-414-4H w. (2) CP441-2 (RS485 at slot IF1) for redundant Modbus communication to refinery control system (Honeywell). System was running without any Modbus errors for 3 years.

After adding (2) CP341 in the redundant periphery for non-redundant communication with two separate systems, Honeywell-Control-System sometimes receives the return value 84 after sending FC05-"force single coil".

The new installed Modbus connections are running without any problems.

As this problem occur only sometimes, we do not know how to solve this problem. Seems as there is a problem located in Honeywell, but how to show this our customer? We only have Modbus testing software at site...

Who can give us a clue?