LMDE XFCE : no more video after update pack 6

[Display]

I have installed the update pack 6 (without error) and I can no longer play videos with VLC, Totem, or other.
The sound is ok, but the video is just black.

When I go to youtube, the video is ok inside the browser. So I suspect a codec problem.

I don't know where to start to solve this problem.
Any idea ?

[Display]

I found the first part of the problem : the video applicatins are set to use "default" as video output. This currently gives a black screen.
If I switch to "x11' for example, I then have video. No more black screen.

I have to set "x11" for every application. Where can I find the general settings ? So I can use "default" for every applications, even those without this kind of setting (Totem for example).