Pictures from Media Library do no longer show on Website

I tried to exchange a linked picture on a site today. On the website it is the picture to the left. Uploaded the image to media library, exchanged link and id in the code, but it won’t show. On one of the sub-sites I tried to do the same with the visual editor instead of code (sidebar, second pic: https://bockvonwuelfingen.org/trainings-fuer-leitungen-teams-gruppen/), but it won’t show either. I can see it in the visual editor preview, but as soon as I activate the actual preview or activate the code, the pic can’t be displayed. It also kicked out another picture on the bottom which I didn’t touch at all. I feel like it’s a media library related problem, as it’s also a little buggy when I upload new pictures.

Played around a lot, but I also can’t link back to previously uploaded pictures either. Any ideas?

This site is individualised with snippets of HTML and some custom CSS. However, I did not do specifications about the images themselves (size, id etc.) so I hope it’s not related.

(The problem started today, I did not encounter this before. It persists after updating to 6.4 and disabling recently added Plugins, so I don’t think it’s related.)

Thank you very much in advance for any input!

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

 

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