Now look at this:

 $ sudo apache2ctl -D SSL -S
 VirtualHost configuration:
 131.246.124.231:80     is a NameVirtualHost
        default server ofset6.unix-ag.uni-kl.de (/etc/apache2/sites-enabled/000-default:2)
        port 80 namevhost ofset6.unix-ag.uni-kl.de (/etc/apache2/sites-enabled/000-default:2)
        port 80 namevhost cvs.ofset.org (/etc/apache2/sites-enabled/cvsweb:7)
        port 80 namevhost gnuedu.ofset.org (/etc/apache2/sites-enabled/gnuedu:2)
 131.246.124.231:443    is a NameVirtualHost
        default server cvs.ofset.org (/etc/apache2/sites-enabled/cvsweb:34)
        port 443 namevhost cvs.ofset.org (/etc/apache2/sites-enabled/cvsweb:34)
        port 443 namevhost gnuedu.ofset.org (/etc/apache2/sites-enabled/gnuedu:35)
 Syntax OK

Do you see the issue now? Yes? Good! Otherwise:

This line

 default server cvs.ofset.org (/etc/apache2/sites-enabled/cvsweb:34)

is wrong. For the real, default server name is ofset6.unix-ag.uni-kl.de.

My 'default' vhost configuration was wrong, I had forgotten to write an SSL paragraph (:443) in the default vhost configuration file. That was it. Now I have

 $  sudo apache2ctl -D SSL -S
 VirtualHost configuration:
 131.246.124.231:80     is a NameVirtualHost
        default server ofset6.ix-ag.uni-kl.de (/etc/apache2/sites-enabled/000-default:2)
        port 80 namevhost ofset6.ix-ag.uni-kl.de (/etc/apache2/sites-enabled/000-default:2)
        port 80 namevhost cvs.ofset.org (/etc/apache2/sites-enabled/cvsweb:7)
        port 80 namevhost gnuedu.ofset.org (/etc/apache2/sites-enabled/gnuedu:2)
 131.246.124.231:443    is a NameVirtualHost
        default server ofset6.ix-ag.uni-kl.de (/etc/apache2/sites-enabled/000-default:51)
        port 443 namevhost ofset6.ix-ag.uni-kl.de (/etc/apache2/sites-enabled/000-default:51)
        port 443 namevhost cvs.ofset.org (/etc/apache2/sites-enabled/cvsweb:34)
        port 443 namevhost gnuedu.ofset.org (/etc/apache2/sites-enabled/gnuedu:38)
 Syntax OK

and it works .. :-p