I had complaints from some users with long forgotten updates who ended up with the same symptoms.
The easy way, rename your home/user folder (LiveCD) and install the latest and greatest.
After reboot copy the important parts from your renamed folder back to the new home/user
These are the folders like Documents, Videos, Musik, Pictures, .thunderbird, .mozilla and whatever in de . settings folders is important for you. To view the folders with . , they are hidden, use Alt-. (KDE) or Alt-h (Gnome)
Repaired/installed those garbled machines in about 20 minutes without searching for the why and how
Updating, Localization, LibreOffice and more fine tuning all within the hour. (Try this M$)