We're in the process of migrating from Exchange 2010 with Appriver over to Office 365 (without Appriver). After digging into migration options, Migration Wiz rose to the top for being easy to set up, use, and pretty darn quick.
All mailboxes in the connector are configured exactly the same way, from Exchange 2010, through the connector, to Office 365.
Somehow, about 1/3-1/2 of the mailboxes either won't start migrating or will error after a while and refuse to restart giving the error message attributed to the source server:
Your migration failed. An internal server error occurred. Try again later. Error:ErrorInternalServerTransientError
Migration Wiz's knowledgebase is a quick article on the error:
https://community.bittitan.com/kb/Pages/ErrorInternalServerTransientError.aspx
Seems to suggest that the originating server(s) with Appriver...