Ok, every plug-in is up to date.
I deaktivated all plug-ins except “cornerstone” and “woocommerce” as without these plugins the site is not loading properly due to php errors.
Still, the “gap” is not shown in cornerstone. I mean, not even in the builder view. You see I am in the menu of a gap, but it is not shown:

Using the original theme and not a child made everything work well again…hmmmm
After coming back to my child-theme…everything works as normal…
So now I am turning on my plug-ins step by step.
It seams as WP Rocket is the enemy…