Ubuntu 18.04 to 20.04 - Unresolved issue occured while calculating upgrade

I had a very similar problem. main.log was identical, the problems listed in apt.log were different.

In my case the culprint seemd to be Qt4 together with Python. I solved my issue by removing these from my system:

sudo apt-get remove python-pyqt4 python-pyside python3-pyqt4

Id did not have python3-pyside installed so if you still have troubles after removing the above mentioned packages, you can chack if python3-pyside is present on your system.

Do not forget to do a apt-get autoremove at the end.

After that I was able to start do-release-upgrade without the error message.

As before you upgrade Ubuntu, you have to remove unnecessary and corrupted packages. I have resolved issue by following commands:

apt list --upgradable
sudo apt autoremove
sudo do-release-upgrade -d -f DistUpgradeViewGtk3