You were right Till, I changed listening port to 8080. I put it back to 80, with my router forwarding external port 8080 to internal 80 of my server IP.
So, being with EasyDNS, I switched off DNS-Server. With EasyDNS panel, I created A record "server1.example.com" that has an URL to "https://IP-adress:81/" and A records "user1.example.com" and "user2.example.com" with IP.
The ISPConfig on "https://server1.example.com:81/" respond from the outside without any problem.
I created the two clients and their respective websites.
When I type address "http://user1.example.com:8080" or "http://user2.example.com:8080", I'm redirected to "http://user#.example.com:8080/apache2-default/" ...
Is it a DNS problem?
I expected to see the user1.example.com page with the blue stripe.