@martin: What a pity. It works like a charm.
I read the following:
Difference between SSH and SSL | Difference Between | Difference between SSH vs SSL
SSL means “Secure Sockets Layer”. Many protocols — like HTTP, SMTP, FTP, and SSH ‘“ were adjusted to include the support of SSL. Basically, it works as a tier in a certain protocol to provide cryptographic and security functions.
The combination of certificates and SSH is very powerful. There is no need any more for users to copy their public keys to the server. The Certificate issuer (CA / signer) can make the certificate valid for a limited time period. SSH options can be allowed or disallowed per certificate. It's great.