The message "Invalid PLIST" is misleading. RETURN CODE 20 REASON 31 problem is an Event overflow It indicates MAXXREQ table ran out of room to handle remote IMOD requests.
The recommendation is to increase the MAXXREQ parameter in GSS to 8000. Maximum value allowed is 10000.
Also it is recommended to increase the EVENTS parameter to the max of 32767.
The combination of these two higher values in GSS should correct the RETURN CODE 20 REASON 31 error