Time Machine couldn't complete the backup - The disk is already in use
Timemachine backup failed after upgrading to macOS Big Sur with 112 error
Please check if the macOS installs Sophos Home.
Time Machine backups to NETWORK locations are currently failing when Sophos Home is installed on Big Sur macOS 11 - Users will see a message reading "Time Machine couldn't complete the backup - The disk is already in use". This issue is currently under investigation by our development team.
"As a workaround, run the backup in Safe mode. We expect this issue to be resolved by March of 2021.", said Support for macOS 11- Big Sur – Sophos Home Help