The real reason for that error was that my router from the ISP didn't forward the port 443... simply a stupid failure in the configuration...!
However, I found on the way towards the solution this guide for making a server much more secure against known SSL attacks: https://weakdh.org/sysadmin.html
I following that guide and disabled SSL3 in addition. And now my server gets a A+ in the SSLtest of ssllabs!
perhaps one could implement it in a future ijrelease of Nethserver!??! Basically it is only a change of the CipherSuites in /etc/httpd/conf.d/ssl.conf and /etc/httpd/admin-conf/httpd.conf and disabling SSL3 in the former:
SSLProtocol all -SSLv2 -SSLv3