Hi Twinky,
thanks. Even though this is not relevant for me anymore, I meant something else: I did not need self signed, but "real" certificates. I was able to create an export of the needed files, but I did not succeed to make use of them.
Fortunately, I am back on a hosted server with a Let's Encrypt certificate now.
Thank you for your reply anyways. Maybe someone else can make use of our talk.
Olaf