Shortly, I have a small chunk of html-code visible at the top of my website, on some of the pages. For instance: http://siebepalmen.nl/videos/, where you can see the code and a link “Continue Reading”.
It is outside the page and therefore not removable. Do you know where this comes from and how to get rid of it?
Indeed. This is caused by your open graph meta tags in the HTML head. The value of the og:description meta tag is not properly escaped and it actually outputs HTML tags which breaks the HTML code for your page(s).
Are you adding the open graph meta tags using a plugin? Please check your plugin setup and make sure the values are properly escaped and HTML is removed from those values. Or contact your plugin developer for help with this matter. Or use a different plugin :)