Calling LoadLibraryEx on ISAPI filter DefaultAddonFilter.dll failed

I recently had an interesting issue with a client’s CRM 2011 to 2016 go-live. Following all of my upgrade work and initial testing, I typically like to reboot the CRM and SQL servers and then test again. In this particular instance, when the servers came back online I tested CRM again only to receive a lovely error message from Internet Explorer:

HTTP Error 500.0 – Internal Server Error

Calling LoadLibraryEx on ISAPI filter C:\Program Files\Microsoft Dynamics CRM\Server\bin\DefaultAddonFilter.dll failed.

After a fair amount of digging I noticed that the Visual C++ 2013 Redistributable (64-bit) was not present in the add/remove programs list on the CRM server. It seemingly uninstalled itself during the reboot because otherwise I would have had this issue pre-reboot. So, I went out and grabbed the installer package for Visual C++ 2013 Redistributable (64-bit) from MS (http://go.microsoft.com/fwlink/p/?LinkId=402059) and reinstalled it. After that and an IISReset, CRM began functioning as normal and everything retested everything successfully.

Unfortunately, I am at a loss as to why this happened – perhaps something to do with server patching. Also of note is that the redistributable did not remove itself from the SQL server.