Hi
Mikko Uusitalo,
Welcome to Microsoft Q&A forum.
As I understand, you are doing Azure Database for MySQL Single to Flexible Server migration and getting black screen.
This is strange and should not occur, please check below points:
1). Make sure the DMS service is on.
If the user explicitly stops Azure Database Migration Service (DMS) or if the service is inactive for a period of 24 hours, the service will be in a stopped or auto paused state. In each case, the service will be unavailable and in a stopped status. To resume active migrations, restart the service.
2). Check size.
To support migrations of databases that are 1 TB+, raise a support ticket with Azure Database Migration Service to scale-up the migration agent to support your 1 TB+ database migrations.
In the meantime, I will also try to check this issue by replicating the scenario.
If above does not help, I would recommend you to please file a support ticket for deeper investigation and in case if you don't have a support plan, do let us know here so that we can check on other options to unblock you.
Thanks