I also faced the same 503 error in my all the websites, I uninstalled and reinstalled the IIS web server, but it did not resolve the 503 issue, I deleted the http, WAS and WSVC3 Registry, It got suggested by Grock AI ...but it was my big mistake after deleting registry, it could not be recreated through commands, and Now I faced another issues these services are now showing missing while restarting webserver, I just left every AI suggestion behind, Just checked my all other servers to find same server OS and after trying around 50 server I got the same version OS just exported the above registry keys form this and imported in my problematic server, rebooted the server and magic was done 503 error gone. but While reinstalling webserver it recreated (i previously renamed the old file) appicationhost.config file and only 30 sites came back in IIS they were also having issues like SSL binding, coding issues but after minor changes I made 100 website live, but my client got happy said, no issue I will manage with others, important are live that is enough.