During a migration with a heavy load of clients being installed in a short period of time at a customer I run into this issue with WSUS app pool becomes unresponsive.
One of the error messages were:
WSUS Control Manager failed to configure proxy settings on WSUS Server
https://blogs.msdn.microsoft.com/the_secure_infrastructure_guy/2015/09/02/windows-server-2012-r2-wsus-issue-clients-cause-the-wsus-app-pool-to-become-unresponsive-with-http-503/
