I received reports that users could not login to CRM. They would attempt to open the CRM site and would immediately receive an error. All users, even the Deployment Administrators were not able to open CRM.
The Event View Application log had lots of error with Source: MSCRMKeyGenerator / Event: 18949
Inside these errors I found the following:
Current active key (KeyType : CrmWRPCTokenKey) is expired. This can indicate that a key is not being regenerated properly. Current Active Key : CrmKey......
Make sure that the "Microsoft CRM Asynchronous Processing Service" is started and then restart IIS.
Basically this service is pretty important for a couple of reasons. Long story short, make sure it's started and if it's not started, you'll need to restart IIS after starting it.
If the Microsoft CRM Asynchronous Processing Service won't start, you either have a locked out user account (hopefully you're running as a domain user account and not Network or Sytem), or you have another problem fixed in the latest cumulative update rollup for Microsoft Dynamics CRM 4.0
Reference - http://support.microsoft.com/kb/949844#appliesto
Cumulative Update - http://support.microsoft.com/kb/949256/