Hello,
We have two different IIS servers running two completely different sites. Both are encountering the same issue. The application pool stops and then site shows a 503 error.
In event viewer, I see a lot of logs like this on both servers.
A process serving application pool 'CONTOSOAPP' suffered a fatal communication error with the Windows Process Activation Service. The process id was '4900'. The data field contains the error number.
I've ensured that .NET CLR Version is set to v4.0 and Enabled 32-bit Applications is True.

This began on both machines around the same time. I suspected Windows Update and have recovered the machines to their state prior to the updates. However, the issue remains.