Modern Authentication got enabled on the customer tenant. After changing the BT configuration to use modern authentication from basic authentication, the customer I was able to migrate users.
Then they tried to migrate Mail in databases that had already migrated some data (prior to the modern authentication incident) and none of them will migrate again
All have the error:
Error: Log on Failed 80040111
Error: Failed to initialize
With Basic authentication, NM uses Auto Discover so it uses the value in the target mail box field on the person or mail in document to login into the mail box, so the exchange information doesn't technically have to be filled in and the migration will still login in, connect and migration.
Modern Authentication migrates the same way old release of NM like 4.5 did with NM using a created migration profile call "Migrate" set up on the worker so the exchange information needs to be filled in on the person document in order for use to find the location of the mailbox , hence "matched".
The migration documents where moved back to the match phase, matched so that we obtained the exchange information and filled it in on the person or mail in migration document and then remigrated and the migration kicked off.
It is always a best practice and a necessary step to match the users or mail in database before migration.