Time Machine fails with 'unsupported disk' after Tahoe 26.2

I encountered a problem when using macOS Time Machine: the backup failed.

System: macOS Tahoe 26.2.

The backup environment is a Samba shared disk on the local area network, a standalone disk.

I have been using this environment for a period of time (about half a year), and it has always worked normally, but in the past roughly ten days I have noticed system notifications of backup failure. Clicking the notification only indicates that the backup failed, with the reason unknown. I tried multiple times and it was the same.

In Settings, I deleted this disk and then tried to add it again, but found I could no longer add it; it prompts “unsupported disk.”

There have been no other changes to the backup environment or the computer environment; I have only been using it normally. The only change is that I recently upgraded to macOS Tahoe 26.2; the previous version was 26.1.