I finally found my problem. It was an error from me (Of course!).
Obviously I activated the Captive portal on my WLAN interface without allowing to contact the DNS Server ip.
When they could not get an ip for any site the Captive portal would not show and there was my dead end.
When I finally tried to get a site to show by its ip the portal came up and I knew what I did wrong.
Long story short: Its working now, thanks for your help.