I just downloaded every core that was available, tried to play a game, was thrown back to menu. After restarting the system, I still got the same behaviour. Even selecting a core manually and loading the content manually won’t work.
Here is a paste: http://pastebin.com/0HZgWBxw
What I found is this: RetroArch [ERROR] :: Failed to load GLSL shader: /home/kivutar/shaders-glsl/crt/shaders/crt-geom.glsl.
I got a lot of those “artefacts” where the directory points to kivutar’s home.