After having another go at this with the latest svn build, I finally solved the problem. Or at least I hope I did.
I had the same problem as before with the latest svn release. I changed between different .kpxe and .kkpxe files countless times until I tried something I hadn’t done before. I always used symlink’s to switch between .kpxe and .kkpxe files, but instead of doing that I decided to change Option 67 in the DHCP settings for a change. And it worked!
I’ll try a different client later to see if it really is the fix I’ve been looking for.
Edit: Doesn’t seem to solve the Problem. But I found a workaround. Booting any OS (Live CD is enough) and rebooting solves the problem. I guess it has something to do with the NIC not being shut-down correctly by the manufacturer.