UPDATE: Turns out ALL of my app pools were somehow corrupted. It just doesn't trigger the error and shut down on each one until you actually try to run the associated website application.
On one of my dev machines, I had something odd happen that I can only attribute to the Windows 10 Fall Creators Update. One, and only one, of my local IIS application pools stopped working. Every time I tried to run the associated website, it would fail with a 503 error and the app pool would stop running.
Event Viewer showed only the following "helpful" error message.
The worker process for application pool 'devweb.schedulesmart.org' encountered an error 'Cannot read configuration file
' trying to read configuration data from file '\?<EMPTY>', line number '0'.
And I can only attribute it to the install of the FCU. I was literally working on the application in Visual Studio, then exited VS to run the update. After the reboot, I tried to run the app and it wouldn't work. I have no idea what happened. There could easily have been something else that triggered the error, but I don't know what since I didn't do anything else myself.
The fix, after some troubleshooting, was easy. Just created a new app pool and assigned the website to that one instead.