Guys,
The bastards over at Asus (or Avidemux because they don't handle it well) have an ->> OSD <<- on their ASUS GPUTweakII program and it interacts really badly with this Avidemux and especially VLC videolan player.
In Avidemux it crashes with this message and the UI goes grey. That's it.
It's ruined with the OSD option on on the tool from Asus mentioned above. (Turning the OSD off works, but it's difficult to discover what the cause is, so I'd have thought about this issue >:\
TY for the Avidemux program though!