Ok, that was easier than expected, so answering myself:
As I already used
this proceeding to force SSL on ISPConfig access, which also included the creation of a new self-signed-cert, I just needed to add the symlink "000-default-ssl -> ../sites-available/default-ssl" to /etc/apache2/sites-enabled, and change the cert in /etc/apache2/sites-available/default-ssl (which can be omitted, because it is already preconfigured to use another self-signed-cert from /etc/ssl/certs/).
I decided to add another symlink, and not to change the existing default one, so that phpmyadmin and webmail can be access normally and by SSL.