Dropbox keeps starting on boot though I disable it on Kubuntu

On Kubuntu, if I go to system settings > startup and shutdown > autostart I can see Dropbox (command "dropbox start -i") in the "desktop file" list and I can uncheck them in order not to make them start on boot. It works until I use Dropbox again. Then Dropbox will be checked in that list and it will start automatically when restarting the system, unless I uncheck it again before. In the Dropbox settings I've already disabled "start dropbox at system start" (my translation, don't know how it says in English) but it's useless (while disabling it in the Skype settings was useful to prevent Skype from having the same behaviour: now it's no more in the autostart list at all).

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