You may see HTTP 503 errors returned from a server that is running Microsoft Forefront Unified Access Gateway 2010 Service Pack 4. This condition remains until the worker process is recycled or until Internet Information Services (IIS) is restarted. Additionally, HTTP 503 errors are logged in the HTTP.sys log file that has a status of "Queue Full."
This problem is caused by a deadlock that stops requests from being processed. Eventually, requests back up in the request queue until it is full. At this point, the error occurs for all successive requests.