Thanks for the comment.
I just tried it again. It doesn't make a difference if the triangle is out or the slide ist completely in. The shutter is never blocked. So it is probably a technical problem with the back. I just realised how the pins on the darkslide push in two pieces of metal and make it possible to take off the back or lock it. This bit works, but it doesn't lock the shutter if the slide is pulled out.
Guess I have to try it with other backs to find out if the problem ist with teh back or the rotating part or the body. The postman will bring me a 6x7 back next week. Let's see.