I use six laptops in my work. ALL USE DROPBOX. Five of them have dropbox version 11.4.,426 & ONE has an older version because it had not been turned on for a while. Of those that have the most recent version and when clicking on the dropbox icon in Gmail to attach a file, a screen pops up saying dropbox refused to connect.
On the one with the older version (not updated - because it had not been turned on for a while) IT WORKED JUST FINE ... Please fix this bug in your system and release an updated version. So it is not any setting in the computer itself if it is happening in 5 of the 6 computers. IN MY OPINION, it is a bug in the latest version that they released.