Account migration, blocked instance, vague error message

[ryliejamesthomas]

A user on my instance was trying to migrate accounts from an instance we have suspended. I assume this is not possible (fair enough!, we temp. unblocked to work round that). The problem is that instead of the user being told what the problem is, they get this:

I'm trying to set an alias in order to migrate here, but I just get "Something isn't quite right yet! Please review the error below" with no error message below.

Both instances are on version 3.0.1