Kudos again to @Moritz_Bunkus
Are Messages from Cron: MUTEX_LOCKING or NOLOCK regarding netlogon_creds_cli.tdb Important?
You notice regularly in the system logs messages from cron like this:
tdb_mutex_open_ok[./private/netlogon_creds_cli.tdb]: Can use mutexes only with MUTEX_LOCKING or NOLOCK Failed to open ./private/netlogon_creds_cli.tdb Error while backing up ./private/netlogon_creds_cli.tdb with tdbbackup - status 1
Are these messages important and if not how can you get rid of them?
This is a known Samba Bug. As the file in question is not very important (should be recreated during a restore automatically) the failing backup for this file can be ignored.
But, unfortunately, the originating univention-samba4-backup script stops due to this error and does not produce a proper backup. To fix, Univention has released an update. Please update.
If you have an additionally backup solution up and running you might want to decide to ignore the failing backup here by just ignoring these messages:
To get rid of these messages you can create an ucr variable which just filters these type of messages so they will not appear in system logs.
ucr set samba4/backup/cron/options='2>&1 | grep -Fv ./private/netlogon_creds_cli.tdb'