A very obscure error occurred after a Windows Server upgrade to 2019. Taboo, I know!

When trying to start an application that depended on .NET Framework 4.8, recived the following error:

System.MissingMethodException: Method not found: 'Int64 System.GC.GetAllocatedBytesForCurrentThread()'

The solution was that .NET 4.8 needed to be reinstalled.

Downloading the package from https://go.microsoft.com/fwlink/?linkid=2088631 and reinstalling, plus a reboot, worked fine.

Must have uninstalled itself or not been kept from the upgrade from 2012 R2. It was a non-production machine, so upgrade was easiest option.