Unable to back up to Time Capsule after upgrading to Big Sur

The one big problem that has come up is AV and security software.. otherwise what you listed covers it.

If you have Sophos or Norton (maybe) or some other firewall, extra security, AV .. installed they can/will stop Time Machine working to Network targets. Have you tried to remove them.. not just turn them off.. generally only complete removal works.

Go through this thread and it covers it.

https://discussions.apple.com/thread/252046759?page=5

• I have tried erasing the hard drive in the Time Capsule but the problem continues.

I am assuming this means you didn't just try.. but successfully erased the TC from Airport Utility on the Big Sur computer. And then immediately started a backup from Big Sur computer to the TC before your other computers started to replace their backups. All backups would be erased.

If you merely deleted the backup for the big sur computer that will not be sufficient.

I replaced it with the same brand and series but with a 6TB capacity

Makes no difference. The computer is not directly writing to the disk.. the disk is under control of the TC firmware. It is presented to the network as a network share.

MacBook Pro fails to back up to the Time Capsule and presents an error

How long does it take for Time Machine in the computer to throw up the error? Does it run through all the preparation for a while.. and never start? Does it start and give you a time to complete but never finish?

That will help figure out where the problem is.

Is the TC main router of the network still or is it bridged behind another router?

• Connecting the MacBook Pro to the Time Capsule via an Ethernet Cable, and disabling Airport networking, hasn't helped.

None of this might get anywhere since attempting a backup to TC out of the network with ethernet should really bypass all the problems. It might really be something more significant.

Is the disk in the TC got a simple name like data (no names with spaces please) The main TC should also be short, no spaces and pure alphanumerics.

If that does not work.. then try backup to IP address not TC name.

Sorry I do not have a computer up to big sur yet.. but direct backup to IP is often a way around some network problems.

In Finder.. mount the TC using Go, Connect to Server. Replace my IP with yours for the TC.

Click continue and put in the password for disk access. make sure the TC is NOT using anything other than default password for disk access. i.e.

Once mounted in Finder.. go to the Time Machine add disks.. and you will see the TC listed as IP address.

Select it and see if that helps.