Plesk Migrator from Confixx: Database Migration fails

We're trying to migrate a few hundred hosting accounts from a Confixx server to Plesk 12.5 (running on Debian jessie) with the Migrator Extension.

However the databases are not migrated. Migrator shows:

| `- Client 'web230'
| |
| `- Subscription 'web230.example.com'
| |
| |- error: Database 'usr_web230_2' from mysql server at localhost:3306 will not be copied as no mysql DB server is assigned to subscription 'web230.example.com'
|...

Plesk Migrator from Confixx: Database Migration fails