Ok, today I was programming all day without any problems but suddenly, the first time, I got the message "Error E1242: Can't open video device". So I tryed the -w3d command. Nothing changed. There are two odd things. The first is that it ran well all the time and suddenly without changing anything it didn't worked anymore. The second thing is that it works well the first time I start SED and run my programm, but from the second time on it doesn't work anymore again.

EDIT: Ok, I just found out that the normal testrun mode works fine, Only the debeug mode has this problem

That unnormal too

After your first debug run, but before the second, or after the second,
try opening windows task manager an see if there is a "dead" acknex.exe
running in the processes list.
It may be hogging the directx device....

Best guess Ive got.

I meant after the first. But oddly te problem now vanished after using the normal testrun a few times