If anyone figures out a solution to this, I’m also having the same issue. All of my drivers are fully updated, and my game started spontaneously popping that error around the same time as OP. So far the only fix I’ve found is restarting my computer, but that only works for an hour or so before the graphics driver crashes, I have to force stop GW2, and when I try to relaunch it, I get the “unable to initialize 3D output” error again, and have to restart.
Specs:
Windows 10
Intel Core i7-4700HQ CPU @ 2.40GHz
Intel HD Graphics 4600
2047MB NVIDIA GeForce GTX 860M
Any solutions would be appreciated!