Thank you holch and tpartner for your help!!!
I double postet since i couldn`t find a solution in both forums, my problem is unfortunately not solved yet and thought it might as well help people who don't speak german and read it here.
@ tpartner thank you for your file. The file is working perfectly. I made in analogy all the changes in my survey
1 checked for unnecessary code in the equation
2 wrote "randnumber" instead of "rand"
3 put the randquestion within the first question group (Instructions)
4 and also found, that I always had randnumber =='1' instead of randnumber"1", so I changed that as well.
When I check the Syntax, I get no errors- but still, the preview is not showing me the question groups 2-19.
Do you or someone else have any other Ideas I could check???