I upgraded my installation from Version 3.14.11 to 3.15.6.
There were somequestionnaire running. When I open a running questionnaire and try to jump anywhere with the Question index, it just jumps to the next page.
When I click anywhere in the index, it also just jumps to the next page.
I've found the problem myself.
I developed the questionnaire with a custom template (based on fruity).
As for debugging reasons, I changed the whole questionnaire back to the standard version to fruity, which I forgot.
Now I changed it back to the custom fruity theme and the Question Index is working again.