PROBLEM: The WS-Management service cannot process the request. The user load quota of 1000 requests per 2 seconds has been exceeded. Send future requests at a slower rate or raise the quota for this user. The next request from this user will not be approved for at least XX milliseconds.
This can happen if a certificate on Exchange expires (SSL or UCC) and the IIS service has not been restarted since.
All you need to do is open a command prompt (Run as Administrator if using UAC) and the type in IISRESET. Press ENTER and wait.
Now you should be able to login to the Exchange 2010 EMC with no issues.
Really simple fix.