Sidebar under content only on single post article

Hi to you all,

Here’s my issue:

When i click on an article that i want to read, the sidebar of this page (I guess it’s single post article) appears under the content. When i select the category, there’s no problem. Even on the main article page it appears on the right place. I did not make any modifications to the code.

Please check: https://zwangerschapswebgids.nl/article/example/
to see the problem.

Thank you in advance,

Nina