I should mention I've also checked the BIOS settings, messed with the IPL boot order and controller order in every possible way. I have even reset the BIOS settings on one of the servers with no result whatsoever. All this in addition to trying to boot with the backup ROM versions on all the servers with boot problems.
Also, on one of the affected servers (the one that won't boot from CD-ROM either) I've had some filesystem errors at last successful boot. Maybe this one has some more serious issues...