Hi Hugues!
I tried to change the resolution to 1024 to make a test and the lines disappeared.
the initial white screen with lines was not displayed anymore and only see the extended display.
But
I found a solution!Go to resolution and display menu, and change the position of the 2nd monitor ( projector). Default is upper attached to main display.move it down. Oo. And apply.
It works for me!

Sorry for my poor english :/