This was actually a very simple error to fix once I knew the cause, but the problem is that the Microsoft knowledgebase didn’t give the correct answer and there are no log files to be found to help find the correct cause, things were severly complicated by the fact that every search result I could find on the error insisted that it was a security problem. Finally, I had to just guess, and I was right. 🙂
The problem turned out to be that I installed another app that switched IIS to use 32-bit mode. All I had to do was switch it back to 64-bit (x64) mode and SharePoint worked again.
First, fix IIS:
\inetpub\AdminScripts\cscript.exe adsutil.vbs set W3SVC/AppPools/Enable32BitAppOnWin64 False
Second, reregister the .Net isapi filters:
That’s it! How about some better logging, Microsoft?