What causes TimeMachine backups to fail on NAS devices?

Im having a similar issue. We use a Synology Nas to edit footage from. Its been fine for near on 2 years. I updated (mistake) 2 weeks ago and then all of a sudden monday monring the server is disconnecting whenever it needs to do something ie: Exporting a video, reading the timeline, importing photos. It doesnt seem to matter about the file size. It just disconnects. Any ideas here? its only an issue with the NAS. Other drives like the Black Magic Cloud unit is fine. Also transfer speeds have slow right down

I have the same setup as you. I had to reboot twice after the 15.1 update and I started a new backup (because I thought that the old one might be corrupted). I don't know if the new backup was required but the reboots certainly were. Everything seems to be working well now. Good Luck!

I did this already with 15.0 + 15.0.1 and that fixes it also temporary, but I came back after a while. Will try this as well with 15.1 but this shouldn't be the solution to remove the whole backup and start again.

Disabling the firewall worked for me. I think I recall it working once or twice without deactivating the firewall on 15.0.1.

Now after updating to 15.1, it seems to work reliably.

This is also backup to a Synology NAS.

summary: 15.1 fixes it for me.

15.0.0 and 15.0.1 broke Time Machine backups for me as described in this thread. disabling Apple firewall in network settings was the workaround.

15.1 fixes it. I no longer need to disable the Apple firewall for Time Machine backups to complete on my Symplogy NAS.

Software: Little Snitch Mini and Cloudflare Warp.

hardware: MacBook Air M3 and Synology DS220+ NAS.

So I can back up with Time Machine now I am on Os 15.1. But only triggering the back up manually. If it's on the normal hourly auto. setting, it looks fine but then reverts to the "server disconnected during the procedure" type error log. But at least I can now back up. Definitely not moving from Os14 for other devices.

That because it Disney at every Mac Upgrade each year! That’s why I consider Time Machine to to basic and myself went shareware backups that continue programs instead where it picks up older backup files!

Appears fixed in Sequoia 15.1 (definitely broken in 15.0.1). I am now able to turn my mac's firewall back on. There was also an update to Synology DSM 7.2.2-72806 I installed at about the same time. So between the two updates, it is now fixed.

You're out ahead of your skis. Worse, you're smug about it. It's clear it is not caused by third-party firewalls, or even Apple's firewall - I don't have a separate firewall, and Apple's is turned off, and I still have the problem.

To be clear, it's not clear what is causing the problem.

It does seem likely that you're repeating yourself.

This is what I'm trying. Downgrading back to 15.0 per support page. How to download and install macOS - Apple Support

These are my DNS settings with Cloudflare

Your Welcome

Not using hosts file or so. In my case also proper DNS. As said, before upgrade to 15.0.1 everything was working perfectly.