Overlay jumps to top

Hi,
when I klick on the thumbnail with the overlay-link the content box is loading in the center, but than it jumps to the top.
I used anchors in the linked page, and the lower the anchor is set, the more it moves out of the browser-window.

Please see the website:
https://lieblingsrogg.bplaced.net/kv/#saenger
Click on the picture in the middle and on the right to see overlay.

The problem is not happening in Firefox, but in Chrome and IE.
Please help! It should simply stay in the middle of the window…!

https://www.ads-software.com/plugins/wp-colorbox/