Error 500 at auth.owa page

I had two databases which were acting funky..  I created two new databases and migrated all the mailboxes..  deleted the old databases.

The users who were on those databases are now complaining that they (intermittently, not always) type in their user/pass in OWA, but they get a Error 500 when they are redirected to the auth.owa page.    With their credentials, I'm able to duplicate the problem.. sometimes.  

I've verified that the problem exists on both of our load-balanced CAS servers.  It doesn't matter which DAG member is hosting these new databases.  I even took one user and migrated him to another database (one that has been in existence for a long time).  He still experiences the problem.

Any ideas?  I'm on Exchange 2013 SP1
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MontoyaProcess Improvement MgrCommented:
Is your authentication service turned on?
rcarseyAuthor Commented:
I didn't think Exchange 2013 had the Forms-Based Authentication Service any more.. ?

The problem only happens for users on these 2 databases (and 1 user who i migrated from one of these "broken" databases to a different one).   Users on the other 30 databases can log in without problem.
rcarseyAuthor Commented:
Also, I forgot to mention ..  the SystemMailbox was one of the mailboxes that was migrated to a new db.  can that be the root cause somehow?

I also see these in the Application Event log:

Watson report about to be sent for process id: 28024, with parameters: E12, c-RTL-AMD64, 15.00.0847.032, w3wp#MSExchangeOWAAppPool, M.Exchange.StoreProvider, Microsoft.Exchange.Clients.Owa2.Server.Core.OwaClientSecurityContextIdentity.CreateMailboxSession, IllegalCrossServerConnectionEx4: Client=OWA, ccc39380,
ErrorReportingEnabled: True

Watson report about to be sent for process id: 9600, with parameters: E12, c-RTL-AMD64, 15.00.0847.031, M.Exchange.ServiceHost, M.Exchange.StoreProvider, Microsoft.Exchange.MailboxReplicationService.MapiUtils.GetSystemMailbox, IllegalCrossServerConnectionEx4: Client=MSExchangeMigration, ff07b402,
ErrorReportingEnabled: True

The Microsoft Exchange Mailbox Replication service was unable to process a request due to an unexpected error.
Request GUID: '6df19acc-159b-4dff-972b-70929c28d8a9'
Database GUID: '06d7ea87-e3a6-463f-92e6-3c6fdddd6c09'
Error: Database '72f7e702-d4c3-4678-a0f6-9a04974a28c9' doesn't exist..
rcarseyAuthor Commented:
This problem was fixed with the latest CU.  Apparently what happened was we took all the mailboxes on two databases and migrated them to another database.   Then we removed the original database.   This was a known issue for OWA and was fixed in CU5.

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rcarseyAuthor Commented:
Didn't think this was something that was fixed between CU4 and CU5,  but it was.  There is a KB article about this.
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