TimeMachine Big Sur to Synology NAS

I think I may have figured out what is going on here, although I can't understand the why. After sending Synology the debug.dat in response to the Support Ticket I started I began examining the Connections log in DSM. I noted that each time one of my Big Sur systems tried to access the NAS from TimeMachine there were 2 user connection entries nearly simultaneously - the user I had designated in Time Machine, and another user I had previously designated - both with privileges for that backup shared folder. I went back into DSM and eliminated all those user privileges to that shared folder except the general admin user. I then went back into the Big Sur Time Machine and deselected and reselected the appropriate NAS, entered the general admin user credentials and then initiated a Time Machine backup.

So, in short, have only a single dedicated user in DSM with privileges to your TimeMachine folder, unselect the drive in Time Machine and then reselect it using that dedicated user's credentials. That is what seemed to do the trick for me.

I would just like to know why Big Sur was spitting out 2 sets of user credentials for each attempted logon to the NAS.

A couple of other items I took out of the mix were: (a) removed Sophos AV (which has only a semi-effective implementation to begin with) and (b) turned off Spotlight indexing altogether (just adding the Synology Time Machine share didn't do any good since as soon as you disconnect the drive it disappears from the exception list).

I have left all the settings I had for NAS under Catalina, just uninstalled Sophos and found that my first TimeMachine backup is running fine under Big Sur.

Will reinstall Sophos once the first backup is complete. On the Sophos website they are saying they may release a Big Sur compatible update next week. This may correct the issue. I currently have different users for separate TM backups.

After many hours of work, I was able to SOLVE it on my machine.

I was having this problem (TM error 112) on one upgraded Big Sur machine, while others are backing up fine to Time Machine folders on Synology NAS.

After doing many things (several times), I was able to solve this.

1) Followed these Synology Instructions to make sure it's not Key corruption issue: https://www.synology.com/en-global/knowledgebase/DSM/tutorial/Backup/What_should_I_do_to_fix_the_problem_when_connecting_to_Synology_NAS_using_Time_Machine

2) Deleted Time Machine folder on NAS.

3) Deleted old user and setup a new Time Machine User on NAS.

4) Turned off Sophos Home Anti-Virus

All of that did not allow me to go further than creating a sparsbundle on NAS, with TM Preparing Backup, but then fail in ability to write files to the the NAS sparsbundle.

What finally solved it was:

5) Uninstalling Sophos by downloading Sophos removal tool. https://support.home.sophos.com/hc/en-us/articles/115005499786-Uninstalling-Sophos-Home-on-Mac-computers

Good Luck!

I too found a conflict b/t Sophos AV & Time Machine on Big Sur. Its implementation is substantially diminished in terms of its features, and I don't think it is ready for prime time on Big Sur. So I've uninstalled it from all my Big Sur machines as well.

We like Sophos, but as we have learned it breaks Time Machine on Big Sur. Not even sure Sophos is aware as their latest support document re: Big Sur makes no mention, just all the Sophos AV features that DO NOT work on Big Sur and the system gymnastics necessary to get it to even install and minimally function.

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).

I've been through this pain beginning with Catalina. I have tried all the suggestions on all the posts I found. I can no longer back up to NAS (Synology). It worked great with prior MacOS versions. I've been through deep dives with Synology ("talk to Apple"), and with Apple ("talk to Synology"). A further escalation with Apple (they were very responsive, but the answer is not satisfying) was "per Apple software engineering we no longer support NAS with Time Machine". I was hoping that Big Sur would have a different outcome, but reading this thread would indicate otherwise. Really disappointed in Apple on this one.

Apple is serious about security. This page states that anti-virus is "built in" to macOS Big Sur:

Too early to tell regarding Big Sur but historically, Apple is not infallible:

https://www.macworld.co.uk/feature/mac-viruses-list-3668354/

I am having the same issue. I don't have Sophos installed and never have, but I do have AVG (Free) running. Will this be causing the issue? I have tried the workarounds abiove (users/directories etc) but still fails every time.