Time Machine back-ups stopped the day I installed Mojave

I have removed the plist and rebooted. I have unplugged the time capsule and rebooted. My older mac, still backs up regularly to the same Time Capsule. The things that have been different: I did a full back up to a second drive using USB connection before installing Mojave. I also had installed Sophos anti-virus but have since removed it. I get "preparing back up" but that just goes on forever. And from time to time I will get a reading that says last back up XX days ago, August 1. I have unticked the box that says automatically back up and then reticked it. It seems something having to do with the Mojave update has caused this glitch? One more item, while my back-up Time Capsule is nearly full, it had been routinely deleting old backups as it added new ones. I am at a total loss right now.