Kubuntu 22.04 will not update.

[rikona]

I kept getting these update pop-ups for several days. Backed up
everything, and decided to try it. Clicked on the update pop-up, a fair
amount happened in the background, and finally a tiny message,
indicating there was an error, but no other info.

Decided to do a CLI update and perhaps get more info. Ran:
sudo do-release-upgrade -m desktop

It did do a lot that was visible, but at the end said that the update
failed. Fortunately, it then restored the computer to its previous
state where almost everything was still running OK. That was nice. At
the end of this very long failed update text, it had the following: 
---
Calculating the changes 
Could not calculate the upgrade 
An unresolvable problem occurred while calculating the upgrade. 

This was likely caused by: 
* Unofficial software packages not provided by Ubuntu 
Please use the tool 'ppa-purge' from the ppa-purge 
package to remove software from a Launchpad PPA and 
try the upgrade again. 
---
I tried removing a few PPAs, ran it again, twice, but got the same
result. It seems there is something there that needs to be removed, but
I'm not sure what it might be.

I also took a screenshot of some errors that were reported. These are
at: https://fromsmash.com/cache-error

I also run Claws Mail 4.0.0 that also probably needs upgrading. Could
this be part of the problem since it is not Ubuntu software?

Where to look for the problem?

All ideas much appreciated...

Rik