Symptom:
Finesse Supervisor will see:
Unable to Silent Monitor at this time (CTIError=Invalid Input)

Logs will show:
Mar 21 2014 15:40:04.079 -0300: CCBU_CTIMessageEventExecutor-0-6-DECODED_MESSAGE_FROM_CTI_SERVER<http://www.cisco.com/cgi-bin/Support/Errordecoder/index.cgi?action=search&counter=0&paging=5&links=reference&index=all&query=CCBU_CTIMessageEventExecutor-0-6-DECODED_MESSAGE_FROM_CTI_SERVER>; %[cti_message=CTIFailureConf [status=23,text=com.cisco.Ged188.CRACTIInvalidMessageException: illegal message field lenght: AGENT_ID_TAG]CTIMessageBean [invokeID=565, msgID=1, timeTracker={"id":"FailureConf","CTI_MSG_NOTIFIED":1395427204079,"CTI_MSG_RECEIVED":1395427204079}, msgName=FailureConf, deploymentType=CCX]][cti_response_time=0]: Decoded Message to Finesse from backend cti server
0000003211: 10.252.96.210: Mar 21 2014 15:40:04.080 -0300: %CCBU_pool-6-thread-26-3-API_ERROR: %[error_code=23][error_message=E_CTI_INVALID_FIELD_LENGTH]: Error processing REST API request

Conditions:
UCCX with Finesse as the agent desktop
Agent DN longer than 12 digits