Even i too had the same issue with my SMB CIFS share running in my raspberry pi. I had added my SMB share as part of user login process. and also having a file share discoverable by my mac time machine for backup. After Mac Big sur update it fails saying cannot connect. But i had a different workaround, on time machine if i initiate a backup then trying to connect to SMB works for me. but again this goes away after restart. is the Mac Big sure team working on to fix this issue? i had sent a feed back too.