Time machine backup fails on macOS Big Sur

See https://discussions.apple.com/thread/252056020 for an answer, and possibly https://www.reddit.com/r/techsupport/comments/fzmpy3/time_machine_suddenly_throwing_error_time_machine/

This is a common fail on Big Sur. After spending 2 hours on the phone with Apple support on 14 November I was told that “they are working on the problem” and to check back in a couple of hours. 15 November and still no resolution.

Same problem here... Maybe with the new update will working time capsule backup. Very flustrating, 0 information about this problem here or at another forums...

Time machine isn't working for me as well after upgrading to big sur. It says:

Time Machine couldn't complete the backup to "<HDD-name>"

An error occurred while preparing the backup.