The URL has probably been submitted already once. You can try adding a question mark or a bogus query to re-submit a URL which has been submitted before.
Particularly in this case where pointing to dropbox.com only gets you an error message.
DropBox Twitter Status update: https://twitter.com/DropboxOps
As shown above with Dropbox you might want to use a service that checks for uptime / status (http://www.isitdownrightnow.com/dropbox.com.html)
BTW - landing page is back up again but you can't log in