Those keys access the BIOS. That I know. It's what I've been trying to avoid since it's a great convenience to simply access the boot menu upon bootup, especially since I work with a number of installed bootable drives.
I finally heard from ASRock in the meantime and they've concluded that the board is defective (after having been used without problems over nearly two years). They're in the process of issuing a RMA.
Anyway, thanks for your input.