Kent have blogged about WSUS maintenance before – its important so that we don’t hit problems like the one I’m about to share with you, so make sure you do something about it – Link.
“The request failed with HTTP status 503: Status Unavailable” in this case the source is a stored procedure running (Microsoft.UpdateServices.Internal.ApiRemoting.ExecuteSPGetUpdatesThatSupersedeUpdate).
The wsyncmgr.log snippet above is an example of the WSUS sync failing because the IIS Application Pool had stopped.
The reason for it stopping is that the amount of private memory allowed to the application pool had hit the roof, and when that happens the service stops and nothing works in regards to WSUS. The default limit is set to 1.8Gb, increasing that will allow the WSUS to work through the amount of data as it increases in age and size.
Set the private memory to a value between 6 and 12 Gb and start the application pool again. After that the WSUS sync should go through and everything should fall back into working order.