On using the Dropbox repository for the first time, there are circumstances where you can end up seeing a blank page in the authentication popup window, which doesn't close itself. These are:

• $CFG->wwwroot is an HTTP URL
• $CFG->loginhttps is set to true
• The filepicker was launched from a page which requires HTTPS (e.g. edit profile)

This seems to be because the repository plugin always constructs an HTTP callback URL, so if the opener page is HTTPS the popup fails on trying to access the opener's Javascript, and doesn't reach the window.close() statement (in /repository/repository_callback.php).

As far as I can see (although I haven't checked) the same thing will happen with boxnet, flickr, googledocs and picasa.