Quit the Plex Media Server.
Type the following text into an Explorer window or into the Start\Run textbox:
On Windows XP, Server 2003 and Home Server:
%USERPROFILE%\Local Settings\Application Data\Plex Media Server\Plug-ins
On Windows Vista, Server 2008, 7 and Server 2008 R2:
%LOCALAPPDATA%\Plex Media Server\Plug-ins
Delete Framework.bundle and System.bundle
Restart Plex Media Server, you're all done!
PS: if all else seems to be working but the Plex/Web interface seems to be having issues you can also delete webclient.bundle in step three