Unable to backup Macbook Pro after OS15.3 update

Many reports that macOS Sequoia breaks Time Machine in interesting ways (here's a collation of woes should you wish to feel less alone). I have not yet been able to reliably complete automatic Time Machine backups since updating to Sequoia (including 15.3). Now, once a week, I perform a manual Time Machine backup, and I'll repeat this until a fix is (someday, eventually) issued. Here are the steps:

1. Turn off automatic Time Machine backups.
2. Plug your Mac in.
3. Prevent your Mac from automatically sleeping
4. Open Terminal and issue this command: sudo sysctl debug.lowpri_throttle_enabled=0
5. Manually complete a Time Machine backup. I have found the above command reduces my Time Machine backup time from 5+ hours (ending in failure) to about 90 minutes (ending in success).
6. Return to Terminal and revert the change from your previous command: sudo sysctl debug.lowpri_throttle_enabled=1
7. Revert your setting to allow your Mac to sleep.
8. Repeat weekly until this is fixed. ¯\_(ツ)_/¯

If you're also using a cloud-based syncing or backup service, Time Machine might say that some of the files on your Mac cannot be backed up until syncing is completed.

from:

If you can't back up or restore your Mac using Time Machine - Apple Support



Updated my M3 Macbook Air yesterday, and, subsequently my Linux time machine was no longer available. Frustrating! Finally got it running again by temporarily deactivating the option to block all incoming connections in Firewall Options of the MacOS Firewall (in Network Settings). After that, the Linux server again was shown in MacOS Finder, and, I was able to use the Linux machine's AFP share to run the time machine backup (even after blocking all incoming network connections again).

what happens when you choose "back up now" from the Time machine menubar menu?