Shutdown fails, software and upgrades dont start

[Coming_Curse]

Salve!

Recently I changed the cinnamon-theme and the logout mechanism changed as well. So I changed back and now some things don't work anymore.
- When I try to shut down, the window appears, but no buttons are there. In this case I can start no other application anymore
- the software management doesn't start anymore
- the update manager starts but the area where the updates should appear stays grey so no updates can be made
- while I tried some hints the file manager froze

If I started the terminal earlier the Shutdown command works.

Thanks in advance

Edit: Sorry, I forgot to mention that I use Mint 16 Cinnamon ...

Edit2: To specify one facette of the problem: the freezing seems to occur quite often when I do something with files. Different applications like GIMP freeze or just close from one second to another, when I try to specify a location to save.
I really hope you can help me out here, as this is really annoying and I don't have a clue how to handle it ...

[Coming_Curse]

A part of this problem I could solve: It seems as if somehow some things get written into the additional repositories (/etc/apt/sources.list.d/additional-repositories.list). The only line within this file was 'UFO' - while it could be quite disturbing it is not that mysterious if you consider that I had the dvd for the game UFO aftermath in my dvd-drive
Strange though how this entry (possibly the name of the dvd) came into the additional repositories. After deleting the line the softwaremanager started correctly ...