Error: MigrationTransientException: The call to 'https://hybridserver.company.com/EWS/mrsproxy.svc wstcorpcas01.wilson.com (188.8.131.52 caps:05FFFF)' failed. Error details: The server was unable to process the request due to an internal error. For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the
Looks horrifying right? You have probably tried to use different credentials, created another Migration Endpoint object in Exchange Online, all without avail?
Also you would be scratching your head because you were able to move mailboxes to Exchange Online using the same MRSProxy without issues,
This is probably due to the fact that you have entered the database name wrong:
You probably have databases in a DAG, and when you copied the database name, it ended up something like "database name\server name".
The fact that database names are unique in an Exchange organisation, there is no need to specify the server name.
So what you need to do is enter "database name" into the Target Database field and your migration should be fine.