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I am searching for a solution for this DropBox icon on XUbuntu for many mounts/years. None of the tested solutions worked. No icon, but also no menu linked to it, it's a quite annoying problem.
I had the problem on several machines, with several versions of XUbuntu.
I fact, this solution, found in many places, was nearly the good one, but didn't worked for me... till I have the idea to make a very small modification : put something inside the quote, like "DROPBOX" ! (in fact, any text could certainly be ok).
/usr/bin/dropbox in an text editor (using
sudo). For example on XUbuntu :
sudo mousepad /usr/bin/dropbox
after the line
import os, add this line :
os.environ['DBUS_SESSION_BUS_ADDRESS'] = "DROPBOX"
save, then restart DropBox, or log out/in.
As far as I understand it, this prevents DropBox to find the system notification area, where it doesn't work, and forces it to directly use the main panel, where it works.