Time Machine not working on macOS Tahoe 26.2

There have been reports of issues with TM backups to an NAS, though personally my Mac running 26.2 is backing up just fine to my NAS (a Synology DS223j).

One known issue is when the computer name or backup name contains a special character, the backups fail. For example, MacBackup would work but MacBacküp would not, because of the umlaut. Changing the name solves that issue.

When TM 'refuses to make backups' what actually happens? Does the backup start? Get stuck? Is there an error message?

JMPC wrote:

I checked my Mac's name... it hasn't changed.

The question is, does the name have any non-ASCII characters in it? Those were not a problem with Sequoia and before, but they are with Tahoe. This support article from QNAP describes the issue, but it applies to all NAS TM backups. Note they state, "Time Machine backups to your QNAP NAS fail to complete," which sounds like what you are describing.

https://www.qnap.com/en-us/how-to/faq/article/why-cant-i-use-time-machine-to-back-up-to-my-qnap-nas-after-updating-to-macos-26-tahoe

I was not suggesting the name changed. If there were non-ASCII characters in it before, that would not have been a problem but would be in Tahoe. So in Sequoia, MyBäckup would work but not in Tahoe. As long as the current, unchanged name has no non-ASCII characters, that's not the problem.

Actually, the message you're seeing now is not uncommon with NAS backups in general. For me, at least, that makes it much less unusual and TM reporting a corrupt backup now is consistent with your prior backups were failing.

In my experience, TM backups become corrupt on occasion. The solution is to do exactly as suggested, at the prompt allow TM to delete the backup and start a fresh one.

FWIW, I back up 5 Macs in the house to my NAS. I have seen the above message about once every two years (across all 5 Macs, so extrapolate to once in 10 years on a single Mac). I also back all 5 up to a pair of SSDs that I swap offsite weekly (3-2-1 strategy, 3 copies of data, 2 of which are backups and 1 stored offsite). With the SSDs, I've seen that message one time with one Mac in about 6 years (that extrapolates to once in 60 years for a single Mac to an SSD vs. once in 10 years for an NAS – totally anecdotal, but illustrates the point that they do become corrupt).

You’re welcome! Definitely a good idea to have >1 backup. Also, since you mentioned storing files other than the TM backup on the NAS, keep in mind those should also be backed up somewhere else. Drives fail.

My ‘3-2-1’ is really 5-4-1 because the NAS has two 10 TB drives in RAID1, plus the pair of SSDs. I back up files on the NAS that are not on my internal storage to another pair of SSDs, so those are 4-4-1.