As a preparation to getting rid of Time Capsules we at our company have set-up QNAP TS-133 devices for Time Machine backups. This was working fine under macOS 15 and earlier macOS versions.
After upgrading to macOS 26 Tahoe the Time Machine backups stopped working. Various error messages (Backup is already in use, Backup disk is not available, just after Preparing backup). This happens on all of our upgraded Macs.
The log(1) logfile on macOS shows that backupd process unmounts the backup volume right after trying to start the backup.
Setting aside the non-working backup sparsebundles and starting new backups seems to work fine so far.
What could be the cause here? Backups were working fine before upgrading to Tahoe and stopped working immediately after upgrading. Starting a new backup from scratch seems to work as well (still testing). We can not find anything wrong with the sparsebundles of the non-working backups.
Is this a QNAP TS-133 NAS issue, a NAS settings issue or a macOS Tahoe issue or bug?
Fortunately it is not the NAS but most likely a bug in macOS 26 Tahoe.
The volume name of the no longer working backup sparsebundle is āReservekopieĆ«n van Computernameā.
The volume name of the newly created backup sparsebundle under Tahoe is: āBackāups van Computernameā
This is the hint in the log(1) output that pointed me to the issue being the volume name:
backupd-helper: (TimeMachine) [com.apple.TimeMachine:MountPointValidity] Invalid mountpoint '/Volumes/Reservekopieƫn van Computername' - no volume mounted at this path
After changing the volume name of the no longer working backup to āBackāups van Computernameā and re-adding it to Time Machine, the backup is operational again.
I am not certain if the localisation change from āReservekopieĆ«nā to āBackāupsā in Tahoe is the cause, or that Tahoe does not like the āĆ«ā in the name of the backup volume name.
Glad we have our backup histories back, and hopefully this will help others with the same issue.