Hello Matthias,
To be honest, I am out of ideas too.
I would recommend you ask the moderator to move this thread into SSO space, there are people there who I consider more experienced than I personally am in this area, since we are not getting anywhere with this.
They should be able to help you better, also, I get to learn if there is something that I may have missed.
Sorry.
Regards,
Siddhesh