After trying most of the methods mentioned above, I never got it working. This same issue happened to me after the September 9th feature pack. However, I found a fix that worked for me. After the download, the client ever showed up. However, when I lowered the resolution of my screen from 1600×900 to 1280×720 (resolution I use ingame on gw2), the launcher popped up. I hope this is useful for others.