If we are talking about MC6 Test Stat: MC6 Stat - Space temp point then following is related.
Transcript from modbus documentation:
See here for more info: http://www.simplymodbus.ca/exceptions.htm
03 (03 hex) Illegal Data Value
A value contained in the query data field is not an allowable value for the slave. This indicates a fault in the structure of remainder of a complex request, such as that the implied length is incorrect. It specifically does NOT mean that a data item submitted for storage in a register has a value outside the expectation of the application program, since the MODBUS protocol is unaware of the significance of any particular value of any particular register.
Please see that your multiplier and additive is actually as it should. Device responds to mango that the value you want to write is not allowed.
Edit: I looked at your previous point and it seems that we are actually talking about the same point. If you click on info icon on the datapoint edit page you can see documentation related to multiplier and additive.
The Multiplier and Additive fields can be used when trivial value conversions are required. Numeric values read from the network are calculated as follows: (raw value) * multiplier + additive. The reverse is applied when a numeric value is written to the network.
So generally speaking. You have to consider the multiplier you set to the point and also modbus 2 byte integers take integer values not floating values.