WSUS HTTP Error 503 The service is unavailable

Problem: You receive software update point is busy errors in Software Center, or HTTP ERROR 503 in ConfigMgr client logs, or component status messages in console or browsing to  the SUP website manually.

http://SUP.CORP.COM:8530/SimpleAuthWebService/SimpleAuth.asmx

Message ID: 6703
WSUS Synchronization failed.
Message: The request failed with HTTP status 503: Service Unavailable.
Source: Microsoft.UpdateServices.Administration.AdminProxy.CreateUpdateServer.

Or

HTTP Error 503 The service is unavailable

Solution:

Check your IIS Application Pool and see if the WsusPool is in a stopped state.  If stopped then it’s occurring because the application pool is running out of memory and just stopping instead of restarting.  The Private Memory Limit (KB) for the Application Pool is probably set to the default value of 1843200 KB.  Increase this to 4GB by click Advanced Settings and changing the default to 4000000 KB and restart the app pool.  Keep an eye out for this in larger environments as you may need to increase it even higher if it keeps stopping.