On my laptop install of Ubuntu Server 16.04, when I first installed i3 I found that tray icons weren’t working. I tested with volti, the network manager applet, and dropbox – nothing, and no errors from the applications.
The problem was with the following line in the i3 configuration file (located at ~/.config/i3/config, formerly .i3/config in prior versions [or packages]):
I suspect this would work fine if there were a primary display configured in my xorg settings, but that isn’t the case. Commenting out this line did the trick (removal would also work) after restarting i3 (shift+mod+r).