It is now official. My ISP is stupid: They installed two separate lines for Intarweb access. Everybody has been issued a public IP address, which is OK for me. But the WTF is slowly approaching: Furthermore, two NAT gateways with two different IP addresses were enabled.
This meant: Packets from our public IP addresses were changed so that they originated from one of the NAT gateways. Since the...let's call it "load balancing" was quite random, TCP/IP packets from one stream had two different source addresses. Most servers did not like this. In addition, since there was no port forwarding, SIP/STP did not work, either. Too bad for this ISP, which prides itself with 'VoIP-ready' connections...
The above may look like a rant. It is, in fact, just a comical relief, trying to enhance the (ahem) content of this blog.