Hey Armin, I saw my collaborator contacted you via mail and now it's solved, so many thanks for your time!
For the others to know, I couldn't set the frontpage settings right with that theme-switching-trick because the Customizer didn't work properly, due of a Wordpress bug. Problem was the two urls of the -> General -> Settings were not the same.
I think there is help for this here too: https://wordpress.org/support/topic/non-existent-changeset-uuid/
Cheers!