Hello Duckeenie,
Thanks for the tip but it did not work for me.
However, after some searchs, I know now how to deal with it.
The solution is the following : go to "Startup and session", then to "Application Autostart" and click on "+add".
At "Name", enter "Plank", at "Description", enter Dock, at "Command", enter : /usr/bin/plank, then save and it should work.
Cheers,
tifoli