Can't connect to battle.net

I just bought a new copy of Diablo 2 and Lord of Destruction. I installed it on my Windows 7 (Boot Camp) and tried to play it. But I couldn't connect to the server (Europe). So I opened port 6112 on my router, but still no connection. The error message just says:

Not able to connect to battle.net

So I disabled Windows Firewall but no improvement. Finally I downloaded the newest patch (1.13c) and tried it again. But still it is not working. Any hints on how to improve the situation?

The official support page on battle.net is https://us.battle.net/support/en/article/battle-net-connection-issues and it suggests trying to connect to a different Realm - Europe, USA to see if that works.

The official networking settings page suggests ports 6112 and 4000 (TCP) for Diablo 2 https://us.battle.net/support/en/article/battle-net-connection-issues