sidebar pushes down content

hi!

I got a layout problem on my site: https://www.substrat.imanüm.net.
when the content is posted on a single post (like the image-attachment-page) or when using the search function, the sidebar seems to push down the content of the post as seen here: https://substrat.xn--imanm-nva.net/?s=love

the strange thing is, that this does not happen on the normal “page”-pages (as seen on the homepage)

I tried up and down but couldn’t find any solution