Suddenly I can NOT take Time Machine BackUps on my Synology NAS

The issue isn’t your setup, it’s a compatibility glitch in how Time Machine handles SMB session authentication and Bonjour discovery with some NAS devices like Synology and QNAP.

Here’s what you can try right now, remove your current Time Machine destination, then manually mount your NAS share via Finder using “smb://diskstation.local”, make sure the volume is visible on your desktop, and then re-add it in Time Machine preferences.

Also, confirm that SynologyAssistant has access in System Settings > Privacy & Security > Local Network, and your Mac and NAS names only use standard ASCII characters (no accents or symbols).

If it still fails, it’s a confirmed macOS bug. Report it via Feedback Assistant and keep your Synology firmware updated, since both Apple and Synology are working on patches for this exact problem.

Remove the destination in System Settings > General > Time Machine, then mount the drive and add it back. When prompted, claim the backups on the drive.

Users have reported that having a computer or backup name with a non-standard character causes NAS TM backups to fail. For example, "Backup" would work but "Båckup" would not. If that applies to you, change the name.

FWIW, I use a Synology DS223j with a pair of 10 TB drives in RAID1 to back up 5 Macs in the house and it's working fine for TM backups on a Mac with Tahoe.

Is that the only change you made? There are other settings that also need to be correct, like Bonjour TM discovery. See the documentation from Synology, and make sure your settings all match.

https://kb.synology.com/en-us/DSM/tutorial/How_to_back_up_files_from_Mac_to_Synology_NAS_with_Time_Machine

Personally, I set up using SMB from the start (~3 years ago when it came out), since that was the recommendation at the time.

Looks like a macOS bug.

Reportedly arises with both Synology and QNAP with macOS 26.0.1 Tahoe, too:

• https://community.synology.com/enu/forum/1/post/194663

Related thread here, also touching on the character encoding used by macOS:

• Timemachine Back-up in MacOS 26 (Tahoe) d… - Apple Community

Based on these links, I’d contact Apple Support.

In System Settings > Privacy & Security > Local Network, does SynologyAssistant have access?

After this, I think I'm out of suggestions. Perhaps contact Synology support.

MrHoffman wrote:

Looks like a macOS bug.

Reportedly arises with both Synology and QNAP with macOS 26.0.1 Tahoe, too:

Though clearly not a universal one, since Time Machine backing up my Mac running 26.0.1 to my Synology NAS hourly without any issues. I wonder if it affects those who switched from AFP to SMB?

neuroanatomist wrote:

MrHoffman wrote:

Looks like a macOS bug.

Reportedly arises with both Synology and QNAP with macOS 26.0.1 Tahoe, too:

Though clearly not a universal one, since Time Machine backing up my Mac running 26.0.1 to my Synology NAS hourly without any issues. I wonder if it affects those who switched from AFP to SMB?

Synology with Time Machine server has been the expected SMB with mDNS for a very long time, if not from the onset. Not unless somebody went out of their way to select and use AFP. (As soon as TM got SMB and TM via AFP got noted some seven years ago, TM via AFP was clearly a goner.)

The character encoding might mean it is specific to the characters involved in the string.

So long as the characters match in both encodings, all is well.

If the characters mismatch, bad times ahead.

[This is a guess.]

I did that. No change: TimeMachine claims the NAS doesn’t exist.

Funny thing, though: TimeMachine works like a charm om two MacMinis, it's just my Macbook Air that can't do TimeMachine on my Synology-NAS