The best and quickest and easiest solution is probably what debiman said above. That thread you linked is like 30 yrs old or something. It had the same suggestion in it for fixing the problem. Reinstall.
Everybody wants to help, but if you run 3rd party repos, nobody will want to spend time on it. It's not supported, it's not easy to fix. Even if you get it 'stable' somehow you'll never be sure about it.
The -dmo- pkg themselves aren't the only problem. Sometimes "out-of repo" pkgs can make configuration changes that can persist and cause weird problems for years, or until you find them. You can replace the bad pkg with the debian ver, but the configuration is still there without a purge of every pkg from -dmo- repo. Even after a purge, your configuration in /home remains and that adds more potential misconfiguartion.
If you want a stable system that works, use stable. Stick with stable. When you install something from outside stable, make sure you know exactly what it does, and what files it changes when you install it. Taking notes is not a bad idea.
Relevant to the thread is why you would add this http://debian.c3sl.ufpr.br/debian-multimedia to your sources.list? Why did you do this? Who told you to do it? Why did you not understand that this a 3rd party mirror for dmo that can break your debian system?
I would like to see direct links, and names. How did that get in your sources.list?
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