Migrate backup storage - fails (doesn't start)

Hello all,
I’m trying to migrate to new storage one Urbackup Server running on Windows NTFS.
I’ve created a file named “migrate_storage_to” in C:\Program Files\UrBackupServer\urbackup and restarted the server (file content is only one line: U:\URBACKUP\ )

However the migration doesn’t start.
I have the following error in my server log file (not sure if related though):
WARNING: Error: Unknown action [progress]

Shall it start right away or it will take some time before it kicks-in?

Thank you!