Have you installed IE9?
Microsoft: are still researching this problem. It has turned out to be a bit more complex than originally anticipated. Exchange can not really "fix" this in any way as it is a problem in interaction of IE9 and MMC itself (which Exchange uses as a client). Long story short, right now MS do not have any new workarounds other than
Remove IE9 and wait for a resolution.
You'll just have to open task-manager and kill the mmc-process. Nothing bad happens your changes are saved anyway.
