Finally got my server up and running again. And I know why it was all b0rked. When they switched the IP adresses, I configured it wrong. Instead of putting the old IP on the alias interface eth0:1, I put it on eth0:0, which is the real interface, which confused the server quite a bit. And when I wanted to shut the old interface off, I took out the real one with it. Duh! But now all is fixed thanks to first class phone support from the fine folks at Siemund Online