@Tuny @Nanna_Agesen @Jostein89 do you see any log message indicating what parameter is wrong? (See above message from J-N-K)
EDIT: I think it might be the writeValueType, int16 should be used instead.
EDIT2: further history into why write Value types have different values than read [modbus] Write data types should parallel read types · Issue #4148 · openhab/openhab-addons · GitHub ideally we would like to alias many textual values into one, without polluting the UI