CCC prompted me to upgrade to the latest WHQL driver version (15.7), so I did. After finishing the update my left (main) monitor goes blank. After some fiddling with the settings I managed to get the monitor to turn on again by disabling CrossfireX. Turning CrossfireX back on makes the main monitor go blank again. Probably this has something to do with the main monitor being connected to the "secondary" card due to insufficient DP ports etc (the primary / secondary card is determined by the mobo layout, and as the card with all the displayports is a 3-slot card I can't put it in slot 1 because of ventilation issues). I will now try to rollback to my previous drivers (whichever those were), but hopefully you will be able to fix this issue for the next WHQL drivers.
these are my gfx cards, currently connected in crossfire:
PowerColor R9 280X
ASUS 7970 DirectCU II (the b*tching monitor is attached to this one)
the motherboard is a "Asus P8Z68-V Pro/GEN3" I believe, and I have a i5 2500k cpu attached to it. 8GB system ram. etc..