My system, being a work laptop, has a proxy configured. With the proxy configured, I could not link the dropbox windows app to my dropbox account. When I clicked "Sign in" the dialog box would open, but it remained empty -- not asking for my dropbox credentials.
I disabled proxy settings in dropbox. That didn't help.
It wasn't until I disabled the proxy at the OS level and rebooted that I was able to get connected properly.
Then I re-enabled the proxy and rebooted again. Dropbox wouldn't connect.
I checked the proxy logs and found that dropbox was using the proxy, and being denied:
As soon as I added an exception to the PAC (proxy auto config) file for .dropboxstatic.com, and restarted dropbox, it was able to connect. I'm one of the admins ... without that I would have never gotten it working.
So ... dropbox is ignoring its own config to NOT use a proxy, presumably it is noticing the system environment variables and using the proxy anyway even though I told it not to do so. (HTTP_PROXY and HTTPS_PROXY are defined by Windows, URL points to the PAC file)