I had the same problem. If you are using Sophos on Big Sur, it prevents Time machine backup on NAS. Sophos still has issues on Big Sur. Try uninstall Sophos and reboot.

I used Sophos 10.0.1a1, Ubuntu 20.04 and Samba 4.11.6 with ext4 drive, and I had error 112. I tried to disable scan etc first, but no luck. So I had to uninstall it. Safe boot may work.