What causes TimeMachine backups to fail on NAS devices?

This isn't just NAS's. I have two Mac Mini's and an MacBook Air. The Air is a M2 running 15.0.1 on of the Mini's is a M2 running 15.0. The third Mini is the target and it is older. It is an i7 running 10.15.7. All it does is act as the target and has worked for years until this update. The 15.0 machine works fine the 15.0.1 machine, no dice. I have gone as far as completely reformatting the target drive and reconnecting the Air to it. The connection is initially made and the sparse bundle is created but then the backup goes into the abyss and I get a failure that says the remote disk disconnected.

I am yet to look into the firewall fix that people have been talking about but this as someone said is a complete dumpster fire. Apple has really dropped the ball with 15.0.1. That 15.0 machine isn't getting the update anytime soon.

To add my tuppence worth, it’s more than just backups.

if you search you will find it looks like the Mac’s ability (or lack of) to communicate properly with devices on a local network.

I’ve had issues with RDP connections to machines on my local LAN, VPN issues into my local LAN and have seen reports of app developers struggling to get their programs to pop the “allow access to local lan?” message and I’ve also seen reports of DNS queries failing if the DNS server is on the local LAN.

Agreed it is really terrible and shoddy testing on the part of Apple.

These are not budget devices we invest in and I expect a bug as big as this one to be caught so early in the testing process so as not to appear in the public release.

I have fed back to Apple with my diagnostic logs.

It’s not NAS specific. It’s actually a network communications problem when the internal or even third party firewall is enabled. For me it’s the MacOS internal firewall but I’ve been reading others have a problem with third party.

Reach out to the developer would be my suggestion. I had a problem with my VPN (VPN Unlimited) and they had a new version available via the App Store than what I was running from their website and problem was resolved.

I found a solution for me and Time Machine backups are now working. I had to remove my Time Machine target in the System Settings and had to reset the broadcast setting on the NAS:

After that Time Machine found the Time Machine target again and I could re-attach to the backup and continue. Now Time Machine works again.

Thanks, turning off firewall finally got time machine to work again. Hopefully they fix this.

I use corporate MacBook and cannot change FW settings because it is managed by the company.

Need a fix ASAP!

Hi

What worked for me is to set the Synology configuration as described here https://kb.synology.com/en-global/DSM/tutorial/Time_Machine_backup_failed.

On macOS system preferences I activated Windows file sharing. Now Time Machine Backup works fine.

It not just non Apple firewalls. It the internal one in MacOS as well. I turned it off and the backup runs. Turned it back on and it doesn’t.

Any word from Apple on this? Turning off the Apple firewall didn't work for me, so Time Machine can't backup. I did file a "suggestion" to Apple, but you know how that goes...

Not working for me with Synology NAS and firewall off or on 🙁

Update 29th Oct: After updating to macOS 15.1, I'm able to backup to my NAS with Time Machine without issues (with Lulu and firewall enabled).

Updated to 15.1, tried with and without firewall - but still failing for me :(