Just to share my solution: If you are using the Lubuntu desktop (Light Ubuntu) and Dropbox, you may experience that your Dropbox folder opens in the web browser when clicking on the Dropbox tray icon.
To fix this, you must set PCMan to be the default handler for opening folders.
To do so, open a terminal window (LXterminal) and execute the following command:
xdg-mime default pcmanfm.desktop application/x-directory
Remember: You must not execute this as root!
Good luck and happy Linuxing 😉