Can not upload pictures – HTTP error

Hello dear WordPress users,

I have the following problem: Namely, when I upload an image, then I almost always get an HTTP error / error. I’ve already tried both types (Media Browser) to upload pictures, but it works with certain pictures just not. I have tried everything: converted into different formats, compresses the images, followed some instructions in the network and it still does not work. If I upload a test image from Adobe Stock, then it works, but as soon as I get the picture I can not upload this file because of HTTP errors. I hope you can help me, maybe you will find something on my guide page, what is there, because there are the post pictures are displayed: https://bankenautark.de/girokonto-ratgeber/

Sorry for weird English, I have used Google Translate.

The page I need help with: [log in to see the link]