Big Sur, Big Problems with Time Capsule

I had the same issue; after upgrading both my and my wife's MacBook Airs to Big Sur, back-ups to Time Capsule weren't working. Fiddled with it off & on for a coupla weeks and discovered the issue was most likely due to Sophos (I'd gotten a hint about that from another thread). Deleted & re-installed Sophos again & again & again - same problem. Used harsh language. Didn't work. Prayed; didn't work. Threatened Air w/going back to my Amiga; didn't work. Let it sit after re-installing Sophos on my Air and it magically began to work. Aha! Downloaded Sophos to wife's Air; didn't work - same problems. Repeated harsh language, threats, etc. then noticed on my Air there were five Sophos files under Security & Privacy - Full Disk Access (where you're instructed to drag/drop files during Sophos install) and on my wife's Air, there were only three Sophos files. When I'd searched Sophos Home Free (using DDG), I found two Sophos sites to download from. Both are certified for-real Sophos download sites, but one of 'em downloads a package that gives you the five files and the other downloads a package that only gives three files. Deleted the Sophos on wife's Air, downloaded from the other site, got the five files where they belong and all is well. Sorry I don't remember which URL I downloaded from.....but DDG it, download & experiment until you get the right number of files from the Sophos install package. I'm thinking that very well may have been the issue I was having w/Big Sur + Time Capsule not backing up + Sophos in the mix. Something in the three-file Sophos package doesn't work & play well w/Time Machine & Big Sur. And they all lived happily ever after. The End.

I had the exact same problem after upgrading to Big Sur. I tried everything, in the end I even erased the disk, still didn't work.

In the end I found out that it was the Sophos Antivirus software which caused the problems. After uninstalling it I was able to create a backup again...

With Sophos installed, it works when the connection is 'cabled' This could be a USB hard drive connected or if your computer is connected over ethernet. No issues. Issue comes when backing up wirelessly. So there are solutions if you want Sophos installed or as you say correctly, uninstall and it works as expected. pain in the *ss while we all wait for Sophos to get their **** together.

Ah - another point I forgot to address: I'm backing up over WiFi; I was able to back up to an external USB hard drive with Sophos installed - I experimented with that during my trouble-shooting; 'twas just the WiFi backing up that was a no-go. Once I got the five-file Sophos download installed, I set both computers to back up automatically - I didn't try to manually "back up now" - and then just left them alone to do their thing. They automatically backed up and all's been well since then. I'm convinced it was the two different (why, Sophos - why?) Sophos downloads that I'd stumbled across; one works - the other causes issues. Additionally, I'd slicked my Time Capsule when this problem first cropped up - and that didn't fix it. I also had a 2011 Mac Book Pro on the 'network' and it didn't have any issues backing up via WiFi before or after I upgraded the Airs to Big Sur. The Pro finally died (after years of faithful and zealous service) the week prior to me getting the Airs to finally back up over WiFi. I initially thought that the Airs + Time Capsule didn't like the Pro being involved in the mix. It was merely a data point that I had to explore in the process because of the timing of all of the issues. Had I not noticed the five file/three file Sophos differences, I would be been inclined to think the Pro being on the network caused the issue. But.....nope: I find the different Sophos downloads to be the culprit. For now.........

I was getting the dreaded, "The backup disk image could not be created." - trying to back up to two Time Capsules. 2017 MBP running Big Sur. Was able to create a Time Machine backup using a brand new USB-C external drive. 2018 Mac Mini has no problems with the very same Time Capsules. I concluded the problem had to be with the MBP. Indeed.

Uninstalled SOPHOS HOME, and restarted the MBP. Time Machine backups to both Time Capsules now work.

I haven't read through the entire thread so may have missed something.

Apple will have you power off your router, power off the Time Capsule, restart your Mac, etc and power things back up as a first step.

After that, they will usually ask you to reset the Time Capsule and set it up again.

If still no luck, it's erase the Time Capsule hard drive and try to start over again with a new backup

If you don't want to lose the old backups.......you can copy them over to a USB drive connected to the Time Capsule using the Archive function in AirPort Utility......before.....you erase the Time Capsule hard drive. Unfortunately, no guarantees that the old backups will still be useable though.

I don't use Sophos. I thought I had Avast loaded but looks like I don't even have that any longer, so - no, AV software.

Anyway, since wiping the Time Capsule connected to this MBP now running Big Sur, Time Machine has completed its first encrypted backup about 750Gb, and two subsequent backups. It still seems that whenever Time Machine is Looking for Backup, and for a bit after it is Preparing Backup, the green light for that Time Capsule within Airport Utility turns off, and internet connectivity fails briefly. Otherwise though, now that that TC is formatted vis APFS, the time machine backups ARE working.

I am wondering how to approach my second Time Capsule, which is being shared at the moment by a different MBP and one of my iMac Retinas. The iMac Retina is going to stay on Mojave, but I will upgrade that other MBP to Big Sur also, eventually. When I do, if I wipe that TC, if it ends up formatted entirely as APFS, will the iMac running Mojave even be able to write on that TC's disk any longer? or will only the Big Sur enabled MBP be able to access it?

I finally checked and remove my Sophos Home AV and the Time Capsule backup resumed.

Uninstall Sophos did the trick, I won't use it any more, installed it several times...

Sophos are aware of the issue, and are working on a solution.

They recommend performing back ups in 'safe mode' for the time being.

NB

FYI depending on which chip you're running there are two ways to start in safe mode

Just updated Big Sur to version 11.1 and now the Macbook Air is doing backups as designed.

I installed Sophos midway through the back up and it completely stopped. I've disabled everything I can and its still blocked. Uninstall seems to be the only fix, just now.

Did you.......?

Power off the Time Capsule

Wait a few minutes

Restart your Mac

Power the Time Capsule back up

Try to run a backup

Maybe it is time to give up?

Yes.. I do believe so. It is now old tech.. even if apple only stopped production 2.5years ago, the fundamental design goes back to 2008 with improvements in things like disk size and wifi speed.. but none in terms of the change over from AFP to SMB networking that came with Mavericks??? So it has been left behind for ages.

I noticed that my external drive is still storing in a backups.backupdb. The Time Capsule moved over to a .backupbundle. Does that have something to do with the trouble?

*WARNING.. I am trying to understand myself what is going on.. This is guesswork after reading all the available info.. which as you know.. Apple supplies in a drip feed to those who need to know.. end users are in the category of unwashed masses who get nothing. END WARNING*

The underlying format remained HFS+ even though the drive MUST be APFS. Catalina gives trouble for many people on network backups. There are multiple reasons for this, but certainly the changes in OS protocols to SMB3 (AFP which must be used with TC is deprecated), and continued use of HFS+ when Catalina requires APFS format drive could also enter into it. I do not think the change to .backupbundle was much more than a naming exercise intended to prevent Catalina destroying older backups. Big Sur has now made the full change to APFS on the .backupbundle which was probably intended for Catalina.. but cannot do it without starting from empty target. Check the format on the disk holding backups.backupdb. I think you will find it is now changed.