We do indeed use Agent Handlers (which are used according the mentioned 1 log event above)
I immidiatly upgraded all our Agent Handlers after I upgraded the eposerver.
Even if I enforce to use the eposerver as the source to push the agent from it fails (Then I even get no log events at all )
I also tried removing & reimporting the agents in our master repository.
The issue was related to (corrupted SSL certificate during upgrade of epo server)
Steps 8 ..15 from KB66616 did resolve the issue (generating new certificate).
McAfee KnowledgeBase - ePolicy Orchestrator server backup and disaster recovery procedure