German Compuserve Pro doesn't reach some sites

[informatikoffice]

I use the German Compuserve Pro tariff for using Compuserve for
connecting internet.

I installed everything in the way Compuserve describes on it's help sides.
And it workes. But I can reach some websites only. For example I
reach www.spiegel.de, www.compuserve.de but not www.internet24.de.
But all sites are online. I tried at different times. But it fails. Why?
I am working with NT4, AVM Fritz Modem card, I have a local
network with my own network card. Whrere do I have to set the
network mask 255.255.255.0? I set it in system control/network/
protocols/submask. Okay?

Maybe you can help me. With other ISPs I have no problems.

Thanx a lot
Oliver