Ok I think, I messed up. I upgraded to OS X 10.11.1 before updating the video driver. Is there a way to update it afterwards?
the chicken-egg problem I have now is that I can't update the driver because my video card (GTX980ti) doesn't work without the driver and I can't see the GUI to upgrade the driver x)
The base for my computer is a mac pro 5,1, so I can't do any BIOS stuff or boot into anything else :/ hm… actually I think a linux live cd might work if I remove the boot SSD, but then what? or can I take the SSD, connect it to my macbook using a S-ATA/USB bridge and edit something? :X