While migrating users from an on-premises Exchange environment to Exchange Online, you could regularly see the error TransientFailure in the Status of a move request. Normally this issue will solve itself, except this time. The move request would not proceed 10%. In the end this move request failed with status FailedMapi.

The Underlying error was:

Transient error StoragePermanentException has occurred.

In cooperation with Microsoft Support I solved this problem by following these steps:

Hopefully this will solve your migration problem as well.