The issue i had with crashes has been confirmed to be with a faulty RAM module.
Even though i testet my RAM with the windows 8.1 built-in memtest, and it found no issues, i tried removing 2 of the 4 4GB modules, and found those not to be working. When i tried the others, everything worked pretty good.

Why that would change from before the patch to after the patch i don't know. Maybe the 2.1 Patch demanded more of my RAM, or started using it in a different way.