After recently performing an in place upgrade from Server 2008 Standard to Server 2008 Enterprise (I was after using the DFS features on this particular server) I started receiving an odd error message when trying to load server manager. The full message is
MMC could not create the snap-in. The snap-in might not have been installed correctly.
Name: Server Manager
After the usual research (Bing) it turns out this error relates to something in .NET framework needing a reset after the in place upgrade.
Fix was simple enough, just navigate to one of the two paths (depending on which architecture version of Server 2008 you have installed) and change the file machine.config to machine.config.old and reboot the server.
x86-32 – C:\Windows\Microsoft.NET\Framework\v2.0.50727\CONFIG\
x86-64 – C:\Windows\Microsoft.NET\Framework64\v2.0.50727\CONFIG
Lone behold after the reboot your MMC snap-in will work with no issues.
Do remember however this does change things in .NET and it would be worth checking out any .NET applications on your server after running this fix.
Original solution found here.