Found it!

In case it helps someone, the kernel works when:

- cciss is enabled
- scsi support is entirely disabled
- no other drivers are loaded, such as the Compaq Smart Array driver
(don't recall the name from top of my head, the one just above cciss
in menuconfig)
- MSI and MSI-X support is enabled.

I haven't had opportunity to find out what exactly caused the problems
since I changed several things at once and can't test on this client
server anymore. Unless its a weird conflict with other drivers in
arcane combinations I guess that the problem was not having MSI/MSI-X
support compiled in - although I have been told that it should have
worked without, unless "the hardware or driver in question is somewhat
buggy".