I know, if there's no 80(443) connection to the outside there is no way.
Finally I was successful in solving the problem. My idea in the beginning that there's something wrong to the ip-forwarding in my router was the right trace.
This describes my problem exactly:
(networking - How to make Apache output packets through a certain network interface when connected to VPN? - Server Fault)
I tried to maneuver around the problem by opening a second ip on the server to handle the apache traffic and sending 80 and 443 traffic by ip forwarding of the router to that secondary ip. But unfortunately I found out yesterday that fritz (my AVM-Router) doesn't handle targets with different ips on the same device.
So finally I choosed an other online station to receive the traffic and reroute it to the server's secondary ip and -god thanks- that workaround is doing the job.
So my dear, thank You very much for Your help and patience and as we say in German: it was helpful, that we talked about it