Thank you for trying to help. :)
After converting the movie to AVI, I was able to switch to fullscreen. I made a 9 MB clip of the crashing video if anybody has an idea and he wants to test it.
ftp://free:free@j7n.sytes.net:300/ctemp/crashclip.mkv ftp://free:free@j7n.sytes.net:300/ctemp/crashclip.ogg
FTP may not work in Opera, if it doesn't try
this or
this.