TimeMachine backup error, Again

Try removing the Time Machine entries in Keychain again, then "reconnect" to the backup disk according to the following instructions:

First de-select "Back Up Automatically" to make sure no backups are in progress. Click the Select Disk... button, then Remove Disk, and confirm the dialog box that appears next: Stop Using This Disk.

That action does not affect any existing backups.

Then, physically disconnect the drive from the Mac and connect it again. You might want to restart the Mini as well.

Then, Select Disk... again in Time Machine's Preferences. Re-select the same one under Available Disks, then Use Disk.

See if that fixes the authentication problem.

If you are using Norton security try uninstalling it. Worked for two 13” MacBook pros. Use the full norton uninstall that gets all the files norton hides everywhere. Good luck.

Nevertheless I think you're on the right track. Clearly it's an authentication error and there are multiple links in that chain. One or or more authentication steps are not working. If there are Keychain entries on the Mini reset or delete them as well.

To resolve the discrepancy you may need to expand troubleshooting to your mesh network, which I'm not familiar with.

It wouldn't be the first time I heard of authentication problems with mesh networks.

Please refer to Time Machine troubleshooting on Mac.

... I'm trying to connect via my MASH at home.

I don't know what that is.

Oh boy it's typo.... I wanted to write MESH I was referring to my local network at home.

Ok, I know what that is. I assume your Time Capsule a client on that network?

I don't use any anti virus software, hence why I'm using Mac not PC 😁😂😂