Today NET Core 3.1.200 SDK - March 16, 2020 was installed on my development and production boxes.

With a new release, I tend to also install the Windows hosting bundle associated with each release, and in this case, it was ASP.NET Core Runtime 3.1.2

However, on installing it, the next request to the website showed a 503 Service Unavailable error:

Debugging the w3 process in Visual Studio showed this error:

Unhandled exception at 0x53226EE9 (aspnetcorev2.dll) in w3wp.exe: 0xC000001D: Ilegal Instruction.

Event Viewer had entries such as this:

I tried IISRESET and uninstalling the hosting bundle but that did not help.

I noticed that the application pool was stopped for my website, Restarting it would result in the same unhandled exception as above.

As a troubleshooting exercise, I created a new application pool and pointed my website to that one and deleted the old one.

This seems to fixed things for now.

Success? 🎉