Time Capsules no longer backing up iMacs.

Thanks for the info.

Let's first work on the Big Sur Mac and the Time Capsule connected to that Mac

Hold down the option key on your Mac while you click on the Time Machine "clock" icon at the top of the screen

Select Verify Backups

Start the check and let it run until it completes, or until you see an error message appear

If an error message appears, post back with the exact text of the message

The process might take anywhere from 10-60+ minutes depending on how much data is stored in the backup.

You will normally be notified when the check is complete.

Once you have the Big Sur Mac running the Verify process, you can do the same thing with the Catalina Mac and its Time Capsule.

Time Capsules no longer backing up iMacs

So, you have multiple iMacs and multiple Time Capsules?

Do each of the multiple iMacs back up to multiple Time Capsules?

What operating system is running on each iMac?