Try this:  

	dropbox stop && dbus-launch dropbox start

references:  
- [Dropbox trayicon is missing in KDE](https://forums.linuxmint.com/viewtopic.php?t=216707)  
- [Dropbox icon is not working Xubuntu 14.04 LTS 64](http://askubuntu.com/questions/732967/dropbox-icon-is-not-working-xubuntu-14-04-lts-64/739018#739018)  
- [Tray icon not responding linux mint 17.3](https://www.dropboxforum.com/hc/en-us/community/posts/204590143-Tray-icon-not-responding-linux-mint-17-3#community_comment_206851426)