Okay finally i solved the problem:

for service in domain ntp 80 443; do
$iptables -A OUTPUT -o $WANIF -p tcp --dport $service -j ACCEPT
$iptables -A OUTPUT -o $WANIF -p udp --dport $service -j ACCEPT
$iptables -A INPUT -i $WANIF -p tcp --sport $service -j ACCEPT
$iptables -A INPUT -i $WANIF -p udp --sport $service -j ACCEPT
done

Curl worked - Boinc connected to WCG - everything else started to work - IMPRESSIVE!

Many thanks to all helping me!
Greets
K.

p.s. perhaps my idea to create two reference sites isnt that bad - one with ssl and one without. If the one without https works and the other both not, it must be sth with ssl...