Since Sequoia 15.1, Time Machine does not back up

log out and log in that solved it immediately for me. updated today to 15.1.1 and the problem was back. loged out and in and now time machine is working again. Don't just turn the computer off. Log out while open and log in again.

Cartoonguy wrote:

This has started happening since upgrading from OS 15 to 15.1. I get a message that the backup could not be completed, as below. No idea what "files unavailable means" or

why it stops when the Mac is not awake. Once awake, I resume and it backs up as normal, so what's going on?

https://discussions.apple.com/content/attachment/9631005f-4f2c-4acb-9218-c0fb4effd603

Seems normal

Back up your Mac with Time Machine - Apple Support

> System Settings > Battery>Options and check Power Nap

this will allow your Mac to back up to Time Machine while sleeping

if you have an issue—

Time Machine troubleshooting on Mac - Apple Support

This is an Apple defect that has several variations and workarounds found by the boards. Apple has not offered yet any credible solution or explanation. Apple support and engineering know this from mid-October but nothing has been offered as a solution. The support team at Apple will insist that you upgrade and call them back if it does not work. It won’t.

I tried your solution and it did not work for me. The workaround that works for me is turning off the Apple Firewall. [MacBook 2021 M1 Pro, Sequoia 15.0.1]

Check the threads:

1. What causes TimeMachine backups to fail on NAS devices?
2. Time Machine did not finish backing up because some files were unavailable. Backups will resume when your Mac is unlocked.
3. Since update to MacOS 15.0.1 Time Machine backups do not work