Problem more or less solved. WP-Fastest-Cache the setting Combine Js Plus  Reduce HTTP requests through combined js files (footer) causes the problem.  Check mark out solves the problem. Juiceboxbuilder is displayed again.
Juiceboxbuilder and fastest cache have some conflict?

Thanks for comprehensive answer. Start testing this all in a test environment. I think my IT skills are not quite optimal.
So anyway just as a question.
If I add an image at the end and I change First image index by General to this last image number has this  implications for SEO.
Anyway, the gallery start with the last image added.
This would be a solution for me now without adjustments possible errors occurring in the website.

Thanks,  Problem solved.