In documentation for ArduinoModbus library, all configure function for Server have wrong return value in documentation.
For example https://www.arduino.cc/en/ArduinoModbus/ModbusServerconfigureDiscreteInputs
The function allocate memory for Moddus registers and coils.

Returns
0 on success, 1 on failure 

Return should be ( according to source code )
-1 Invalid argument
0 on failure
1 on success

Same for all configure function