Then that isn't the problem. I measured the corresponding pins on the monitor when it was unplugged, I got about 80 ohms.

Now I have tried running only a 4870 and also only the 4850 that worked before. Now they don't. Driver: 13-1 legacy. I'm beginning to think something in Windows is corrupted. For speed reasons, we deactivated windows update when we installed this machine, and then forgot about it. Now I have re-activated it to see if that does the trick. It will take some time, so I'll try it tomorrow. It's 10:50PM here in Sweden.