Random blank header on posts

Hi,
Since a few hours I am encountering a strange behaviour on my post-type pages. Scenario to reproduce:

1) start from http://fredericbaumann.com/category/savoir-faire/
2) click on any post to view it in details
3) click one of the links in the bottom, under “Articles connexes” (related links), to browse to another post
4) sometimes, the new post is displayed with a completely broken layout: there is one blank page at the top, so you have to scroll down to see the post content. Moreover, the horizontal layout is broken too: text spans on the whole screen width, despite the left sidebar, which actually covers the post text.
5) if you refresh the page, then it displays properly

The issues happens both on Chrome desktop and on mobile.

I was thinking that the issue was coming from these “related links”, but the issue happens also when you browse from the http://fredericbaumann.com/category/savoir-faire/ page (but less frequently…). A change I have done recently, was to try changing the Enfold general layout, to switch from a left vertical menu to a top horizontal menu. It has broken a few things, but I think I fixed them when coming back to the left vertical menu. Maybe some other things should be corrected again?

Please help me, I don’t understand what has happened, and moreover why it happens, especially it is always fine once the page is reloaded.

Thanks!