The postbank uses firewalls with invalid set-up. All ICMP-Traffic is dropped. This causes problems while accessing the site over a Linux 2.4 Router with NAT (The problem background can be found in German here:

Basically, it is the handshaking of the packet size based on MTU of PPPoE by packets with DF-flag (Don’t fragment) set. This handshaking is based on special ICMP command.