jeanbaptisteDV

Great! You saved me.
Wat a useful plugin.
It seems that the only I didn’t check was full-width.php in /page-templates/ folder. And the solution was there.
Thanks again!

Thank you for your answer, WPyogi.
Yes, I created a child theme for having the things done correctly. But it doesn’t help. If I put a new index.php on my child theme, it is not used.
I will get my WP updated to 4.1 soon, but I fear that it will one intervene on any way on my current problem…
Best,