I started having the same issue after installing Evernote on my laptop. I couldn't use Evernote on my PC, laptop or web because in all three, a blank window kept popping up (which never loaded) and closed shortly after, only to pop up again shortly after in an infinite cycle. I assume this was the window to unsync devices, which simply didn't load in all those 3 platforms. Luckily I had Evernote synced on my phone also, and I was able to unsync both the laptop and web apps from there, leaving only the PC and phone. Now it works.