Thank you for your quick answer, this was the easiest solution ever.  

oh..this seems good... Maybe the problem is only just a bug with the section editor..(?)

https://shop.mitenekeljotthon.hu/blogs/adventi-kalendariumthis is the one, I am having problem with. Try to minimize the window. If you navigate to other pages, there will be no problem, only this, and all the blogpost pages are doing this.

header has a blank space under only in mobile version, the more I shrink the page the bigger the space. I only happens on blog posts and blogs. what is the solution?