ErrorInternalServerTransientError: We were unable to connect to the server because it reported a transient error.
Resolution:
Make sure that your server is up and running. If you are able to connect to your server from the Internet, this error may have been caused by intermittent network or I/O problems. Resubmit the migration if it failed, or submit your mailbox in Retry Mailbox Migration Errors mode after the mailbox has successfully migrated.
Some of our customers migrating to Office 365 have encountered this error because they a) forgot to complete the last step of the mailbox provisioning wizard, or b) forgot to assign a quota greater than zero to the mailbox. Log in to the mailbox using OWA and try creating a folder called "test" under inbox. If you are unable to create a folder, check account settings. Also consider recreating the mailbox or contacting Microsoft technical support.