Yesterday, I set up a new Windows computer and migrated Dropbox, which I already used on other machines, one of it on the same network. I logged out on the old machine (because Dropbox Basic allows only three apps running at the same time) and, when installing and authorizing the app on the new computer, which is done from the "Welcome to Dropbox" and Button "Sign In", after entering credentials and completing the two factor authentication code, the Browser became laggy, stopped working and showed a network timeout. I repeated it 10 times with the same result. I noticed that when I access https://www.dropbox.com/home in browser after doing all of the above, I am logged into the web interface though. So far so good. Then I switched back to the old PC which was running before but where I was logged out for hours now, trying a reapply log in there, with the same result. The App does not complete, saying "Waiting for sign in" in tray (I only have the German hover text which says "Warten auf Verknüpfung mit Dropbox Konto".
There is no firewall issue. Browsers tried: Firefox / Chrome. No script blockers, no popup or cookie blockers.