Mylo, I will occasionally get dropped to a black screen (fully du'd, with kdenext). It does not happen often enough for me to replicate the problem. Double -clicking an mp4 in krusader opens VLC, where the video plays just fine. I never tried alt-tab, but I will next time it happens. I have had to shut down each time it has happened. That is the default action for my power button, and it will still bring up the 'shutdown' dialog box where I am able to select 'shutdown' or 'reboot' with the mouse.
It may be totally unrelated of course, but I thought it worth mentioning.