TimeMachine Big Sur to Synology NAS

i was able to solve the problem after update Cisco AMP. With Big Sur and Cisco AMP Version 1.14.0 the TM backup doesn't work. After upgrading to Cisco AMP Version 1.14.1 every works fine.

another MBP with Catalina has no Problem with TM and Cisco AMP Version 1.14.0. The Backup is terminated on the same Synology.

Thanks everyone for solve the problem

I can now confirm that uninstalling Sophos Home and rebooting both my Mac and my NAS solves the problem. I can also confirm that you can install BitDefender on macOS Big Sur, and Time Machine continues to work to backup to NAS. It is the one anti-virus I've heard about so far that does not clobber Time Machine.

I followed all of this and still had an issue. What I found that worked for me was to navigate to File Services>AFP>Advanced Settings and cleared the check mark on "Release resources immediately after disconnect" and finally hit Apply. This kinda makes sense based on what i've been seeing in the TM logs and that Big Sur TM prefers AFP to SMB. I was finally able to get a complete backup of 900 GB in about 5 hours over Ethernet.

I had the very same issue after upgrading a MacBook Pro 13" to Big Sur running Sophos Home and after diving in different forums, I found the explanation on Sophos website : 

Known issues between Sophos Home and macOS 11 Big Sur - Updated December 2nd, 2020

• Since Web Protection is not yet working, expect to see a message on the Sophos Home Dashboard, stating Web Protection is disabled/device is vulnerable - You will also see that the device is vulnerable on the Sophos Home dashboard
• - Web Protection is expected to be functional with the 10.0.2 Sophos Home release - January 2021
• 
  
• Incoming SSH connections will fail if running Sophos Home v10.0.1a1 + MacOS 11 Big Sur.
• 
  
• 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. 

I then started my MacBook Pro without extensions and Time Machine did its backup straight away so it is not related at all to permissions or afp or SMB, it is related to extensions barring the normal process of Time Machine. Backdraw is that you have to restart the Mac in Safe mode everytime you want to make a Time Machine backup...

That is all well and good, but I think the origin of this story (at least for me) was the use of Sophos Home Anti-virus/malware protection with Big Sur, which was causing failure of all NAS-based Time Machine backups. Sophos has acknowledged the problem (see https://support.home.sophos.com/hc/en-us/articles/360051251412-Support-for-macOS-11-Big-Sur-#h_01ESKVJQVKHGVSQFA7E820FX45) and believe a fix will be available in March 2021. In the interim, their so-called "fix" is to perform NAS backups in Safe Mode (not - I just uninstalled Sophos from all my Big Sur machines for the time being, and if this persists . . . well, then Sophos will be no more).

I've been using an external drive attached to an old MacBook Pro as a networked time machine backup drive, and until I removed Sophos, I was not able to successfully complete a time machine backup of a laptop with Big Sur on it over the network. Sophos seems to be the problem, even if the backup drive is not on a Synology NAS. Hopefully they will fix that ...

I just spent a couple of hours on chat and a phone call with a "senior" support rep, and the end result is "Time Machine on Big Sur to NAS is NOT SUPPORTED." You have to use a directly connected drive, so those of us who have invested in NAS technology are out of luck. Tell them what you think about it at apple.com/feedback/

Try what I did by deleting the folder on the NAS that houses the Time Machine backup and create a new one. Also, either designate just one existing user with privileges to that folder or exclude all existing users and create a new one that is the sole user with privileges to that folder. On the Mac quit using the old share as a Time Machine location, then shutdown your Mac and perform an SMC reset (there are various methods depending on whether you have a T2 chip, desktop vs. laptop, etc.). Then on your Mac select the new folder you created above, and enter that sole user's credentials.

Also check to see if you have any Sophos remnants in the Security & Privacy>Privacy>Full Disk Access permissions (I found some entries/references to Sophos that I removed).

As of Nov 25, Sophos added the problem to their known issues:

• Time Machine backups to NETWORK locations are currently failing when Sophos Home is installed on Big Sur macOS 11 - This issue is currently under investigation by our development team. 

Follow-up to this. As I said, I don't have Sophos installed, but I have solved this I think. Simply removed old TM directory on NA. Disabled AFP, enable SMB. Set-up new TM directory with specific user/priviledges. Connect TM to this as new backup source with new TM-only user credentials. This innitiated the first backup, which at 854GB may take some time!! Will report back when I get this done and resume daily backups...

It's an issue with certain third-party anti-virus and anti-malware software. After replacing Sophos Home with BitDefender on Big Sur, Time Machine runs with no problems to my WD My Cloud EX2 Ultra. It's not a problem on Big Sur, per se. It's third-party software that's now incompatible that blocks Time Machine.

I removed Sophos Home in an earlier attempt to fix this. Even with no anti-virus or anti-malware it continued to be an issue. Cautious optimism follows! I have since found a setting in the Synology NAS that seems to have it working right now. I turned off the option File Services>AFP>Advanced>Release resources immediately after disconnect. It's been backing up 30 minutes already without stopping. It makes sense that an AFP setting would make some difference since Time Machine in Big Sur apparently prefers AFP over SMB. That change has not affected backing up from my iMac running Catalina either since it apparently uses SMB to backup.

The latest update to Synology DSM (6.2.3-26426 Update 3) seems to have allowed TM to create the backup on my Syno box now.

TM backup/restore working fine with my 2019 MBPs (macOS 11.0.1) to DS718+ (ext4 format). Changes I made in DSM: <Control Panel><Shared Folders> make a new folder, then <File Services> <Advanced> <Set Time Machine Folders>. When Time Machine is initiated, choose the newly created shared folder <Select Disk> as the backup target. Interesting to note Apple changed the TM backup file name extension from backupbundle (Catalina) to sparsebundle (Big Sur).