I was able to fix the first-mentioned problem by changing /usr/lib/python3/dist-packages/aptsources/sourceslist.py (I attached the fixed version as a patch to this message). It disabled .sources extension to the handled files, causing that problem.
** Attachment added: "sourceslist.py" https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/2055007/+attachment/5749669/+files/sourceslist.py -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2055007 Title: Software & Updates simply does not start, Update Manager crashes when I try to check out the checkbox of an update. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/2055007/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
