Subvolume failed

Hi

When I run manually start_urbackup_server --cleanup x% I got a couple of this error:

TEST FAILED: Creating test subvolume failed

Also many times I got this at the end of the same process:

ERROR: Cleanup failed. Could not remove backups. Please lower the minimal number of backups

Any clue on why this happen and any workaround?
Version 1.4.7

Thanks in advance