When installing Dropbox on a headless Linux machine with their install script (https://www.dropbox.com/en_GB/install-linux) - after install, when the dropboxd script is run a URL to link the installation to an account should be displayed.
This is not being displayed, the dropboxd script just hangs and, using the python helper script, dropbox status shows either "Waiting to link to account" or "Starting".
If the DISPLAY environment variable is set Dropbox will it seems try and open a dialog in X. If DISPLAY is set but does not lead to an accessible X display (X forwarding but no X server on client, stale variable, access control, etc) then dropboxd will hang and rarely display any error output.
Solution: unset DISPLAY before running dropboxd
Note: I say "rarely display" - I did, once, get an X display error 11 hours after dropboxd had been hanging which led me to the DISPLAY variable I was unaware was even set.