Hi guys, I successfully replicated this behavior, but I noticed that participants are able to return via “previous” button, so that this form is evaluated again. Is there a way to disable that, so that the selected question groups stay intact?

If not showing the "previous" button at that page solves your problem, please provide a link to an activated test survey. I can then provide some javascript to hide that button on the given page.

Thanks to @first I added this if around my code, so it does not get executed again if checkboxes already have been marked. Hiding the previous button to prevent users from getting back to this page is not possible, as I shuffle all following question groups by a randomization group. I want users to be able to go back and forth as they want. So this code did the job!

Thanks guys for your quick and great support!