Hello Kristian,
I finally managed to make it work!
The problem was that my source did not deliver the format it was supposed to deliver!
What I did is just trying other sources (such as a PC with a NVidia graphics card to have a good control on the video output (there are a lot of options in the “Nvidia control panel”)) and it worked.
I only tried HDMI for the moment.
Thank you for your answer, and have a nice day,
Hugo