The parameter value was terminated with x'00'. x'00' is not a valid z/OS terminator, even though it was permitted in earlier releases of XCOM. So XCOM is seeing the trailing x'00' as part of the parameter value, because it is not a valid z/OS terminator and therefore takes it as part of the value.
Terminate the value of the parm with a blank right after the value. That will permit XCOM to parse the value specified correctly.
In order to find out what is in the parameter field, you will need to look in the XCOMINIT file for the parameter value specified in the error message and view it in HEX to see the problem.