Dear Richard,
thanks for the reply and ideas. Somehow I was able to fix it by myself and the problem changed in a quiet new and interesting way!
The Frontpage option reapeared as soon as I no longer used a Project Thumbnail on the frontpage. I tried a view more things and in some way the project thumbnails make the pages show up empty in the customizer. That´s wired. As soon as I create a new project and insert this thumbnail to a certain page, this page is not working anymore.
Everything is running up to date and no third-party plugins are running.
Would be nice if you have some ideas for this issue.
If you still want to get access to the admin-panel, please let me know.
Best,
David