Hello again everyone.
Just want to talk about a little gotcha that might occur during your migrations.
You on premise migration endpoint requires on premise credentials to access the environment. I recently changed my admin credentials in my lab and started running into this error when trying to move some mailboxes.
Real descriptive Microsoft. Thanks for that, what would we do without you.
In all seriousness you can receive a more detailed error by using powershell as seen below.
You have to specify an endpoint as you can see. If you do not know your endpoint name you can just do a get-migrationendpoint.
Now that you know you have an auth problem you can fix this by going into the Exchange Online EAC then navigate to Recipients on the left hand side > Migration tab across the top > then the ellipses (the ... button).
Once there you can double click on the endpoint and in the next window you should see a field that says Associated Administrator. Right next to it, very sneakily placed, is the blue Update link we want.
Once we choose that a field to update the username and password will appear and we can enter known good credentials.
Hope this helps someone find what they are looking for sooner!
Thanks again everyone.