Came across this the other, when moving mailboxes from Exchange 2010 to Office 365.
The move works, but takes a hell of a long time. If you look in the move logs you see:
Transient error MrsHttpInternalServerErrorException has occurred.
It would appear that UAG has a limit of some kind that is causing these errors.
I have not tried this, however Microsoft suggest you can try create following registry key on the UAG servers.
“HKEY_LOCAL_MACHINE\SOFTWARE\WhaleCom\e-Gap\von\UrlFilter\InconsistentCookieThreshold” DWORD Value: 30 (Decimal)
Microsoft suggests that this registry key should be removed from the servers, after completing the mailbox migration task.
Activate the UAG server configuration after applying this registry key and then do “IISreset” on all UAG servers.