All Agent to Server communication fails on the impacted Agent Handler, and potentially on the ePO server itself. Typically, the 503 server is busy messages display in the client-side log:
 
The Server_server_name.log (ePO_install\db\logs) file records messages similar to the following:
 
Perfmon counters collected on the ePO server, specifically for Completed Agent Requests and Open Agent Connections, show that the Apache service is completing zero requests.

This issue is different from most max connections issues. Typically, these issues get identified by reduced, but still greater than zero, completed agent requests.

The flat lined Completed Agent Requests suggest that the Apache threads are stuck and unable to process any incoming requests.

NOTE: Other symptoms that might be observed on the ePO server are max connection errors within the server.log. But, the mere presence of these errors doesn’t guarantee that the solution described in this article applies. To confirm that this article applies, implement the steps in the workaround. If the workaround is success, this article applies.