After upgrading to ITMS 8.6 RU2 noticed many errors in the Notification Server (NS) logs like the following:
"Unable to process client request from: <IP address>/POST/8.5.5723.0 (Agent is not trusted., 0x80077003)"
Found several machines that were no longer in the database and were trying to communicate with the NS.
ITMS 8.6 RU2
Was able to identify and find the computers and then needed to either remove the old Symantec Agent or upgrade them to the latest Sym Agent.