Windows 10 Fall Creators update was not yet available on my PC, so I manually pulled the update. The upgrade seems to have run fine except that when I tried starting one of my development services hosted on IIS, it did not. Instead I saw Service Unavailable HTTP Error 503. I checked the application pool assigned to this web site and it had stopped.
Under windows event log I saw this
The worker process for application pool <Pool Name> encountered an error ‘Cannot read configuration file’ trying to read configuration data from file ‘\\?\<EMPTY>’, line number ‘0’. The data field contains the error code.
I knew for sure that the update had caused this issue as I was working on this particular web site just before the restart prompted me to close down my work.
I started looking at the user account under which I was running the app pool. It checked out fine. Next I just went and cleared off all the files under the Inetpub\temp folder. After restarting the services the web site came up without fuss this time.
I got curious since I had no idea what had caused the issue in the first place and started searching for support articles and came across this Web applications return HTTP Error 503 and WAS event 5189 on Windows 10 Version 1709 (Fall Creators Update)
This explained why I was facing the issue though the error message and the event logged was different. You also require to stop W3SVC service which the article missed(?) without which some files cannot be deleted and Remove-Item fails.
Stop “Windows Process Activation Service” and “W3SVC” service and clean out (delete) all the files under C:\Inetpub\temp\AppPools*. Start your services and the sites should be back to work.