Since the last update I made on xUbuntu 18.04 LTS, I got a bug with the file manager (I am using nautilus) that was refusing to open files (only directories opened).

After a few research, I found that this bug has already been identified and is related to new Xfce settings. It is solved by removing (or commenting) the line x-scheme-handler/file=exo-file-manager.desktop in the following files:

• /usr/share/xubuntu/applications/defaults.list
• ~/.config/mimeapps.list
• ~/.local/share/applications/mimeapps.list