I've been having this issue since I built my computer a few weeks back - everything else seems to be working just fine, games run beautifully except Quake Live.
I sign in, try to go for a practice game, wait the 15 seconds for it to start and then:
The weird thing is I can play the tutorial levels. So when Crash is walking me through the different items and stuff, all works fine... except when she says we're going to battle, it counts down from 10, and when it reaches 0, the game crashes.
I've tried it in all browsers + uninstalling and re-installing each time + QLPrism.
I'm running Windows 8. Intel i7-4770K, 16GB RAM, GeForce GTX 760s in SLI.