Unable to start a migration at Tools & Setting > Migration and Transfer Manager. The following message is shown in Plesk Migrator UI:
Unable to execute command because another migration is in progress
The "Starting a migration" step is hung.
Another migration process is running.
Plesk Migrator processes are stuck.
Temporary files left after previous processes. Temporary files can be present if the process was crashed or "killed" by a user and thus a cleaning up was not performed properly.