macOS Tahoe and QNAP backups

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 shows that backupd unmounts the 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?

Ernst Mulder