If you move a mailbox to a new database then immediately delete the source database or immediately decommission the old server users may receive 500 errors when logging on to OWA.

When users try to log in to OWA, they receive either a blank page or an "HTTP 500" error message. For example, they receive the following error message:
The website cannot display the page HTTP 500

This problem occurs because a user's mailbox database GUID (source) is contained within a client-side cookie and is added to the server cache on the Client Access server (CAS). The HTTP proxy on the CAS tries to locate the mailbox database by using the old database GUID within the cache. Because the old database GUID has been deleted, the attempt fails and returns a DatabaseGuidNotFound error.

To avoid this problem, do not immediately remove the source mailbox database after the mailbox move is complete.

To work around this problem, delete all client cookies, and then restart the MSExchangeOWAAppPool and MSExchangeSyncAppPool application pools to clear the server cache.

More details can be found on this link: http://support.microsoft.com/kb/2958434