plesk migrator should plan the scheduled tasks at the end of everything
the cron could cause putting stuff in the database while the mirgrations of very large databases is running.
Bludau IT Services commented
i had migrated one server to another to split some servers.
While the backup is running the scheduled task are added and ready to run every 5 minutes. They grab some RSS and put it into Database.
Maybe this is causing a Database Error.
it looks better if scheduled task are one of the last steps to complete the migration.