It turns out, that the bug I posted is a duplicate of this one:
https://bugs.maemo.org/show_bug.cgi?id=7665

There I also found a workaround for the problem:

Use the terminal to edit the file /etc/resolv.conf. There you find the line 127.0.0.1 (which means somethin on the device handles DNS requests). Comment that line out and add any other DNS Server, for example CCC Berlin's 213.73.91.35 or google's open DNS server 8.8.8.8. So your resolve.conf looks like

Restart you connection and everything should work...