Yes I know OSX 10.8 is old. But it’s a matter of money to buy a new Mac server at the moment.
But you helped me a lot!!!
Applying “ls -al *.pem” I saw that the private key - file had 0 bytes.
So I transferred a new one to the server, now it works.
(I have to use the certbot on my iMac here (10.14. 
))
So thanks a lot, never had the idea to check it.
Michael