There is a sideways scroll bar on one of my blog posts, how can I remove it?

There is a sideways scroll bar on one of my blog posts, how can I remove it? Check the video. 

It wasn't there before, and it seems to be because of this next blog navigation, that is impossible to edit. 
I can add custom css: 
 

html {
    overflow-x: hidden;
}

But that basically cuts the page. 

Isn't there a way to get this fixed without custom css? Whay is it happening?? Why doesn't the blog navigation (that is impossible to edit and delete) just fit the screen?

Any help is greatly appreciated.