I had the same problem. In my case I'm not using zfs so I added acl_xattr to my smb.conf and it works.
[global]
	vfs objects = acl_xattr catia fruit streams_xattr
I don't have much knowledge about acl and so on, but I guess some acl information is necessary for Big Sur's timemachine to work. Anyway thank you so much for your information, ScruffyDan.
dcato
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution