Hello René,
this is due to the “Fix: don’t add the full size image to the srcset if the image is cropped” added in the 1.7 version.
On your page the image only a small image (620x400px) is presented. Of course the zoom cannot add pixels to the image, therefore it needs a bigger image, normally the full size image, to show the zoom. The plugin identifies when the full size isn’t loaded and tries to load it. But this was leading to other problems for smaller images that are cropped with a different ratio than the full size image. Your small image is 620×400 (a 1.55 ratio) and the full size image is 3720×2053 (a 1.81 ratio).
The simplest method for you to fix this is to open the page’s editor, switch from Visual to Text mode, and modify the code from:
<div class="poster-bg"><div><p><img class="zoooom aligncenter wp-image-218 size-zoom-thumb" src="https://beta.emanuelmathias.com/media/Schulungsprogramm_2008-620x400.png" alt="" width="620" height="400" /></p>
to:
<div class="poster-bg"><div><p><img class="zoooom aligncenter wp-image-218 size-zoom-thumb" src="https://beta.emanuelmathias.com/media/Schulungsprogramm_2008-620x400.png" data-zoom-image="https://beta.emanuelmathias.com/media/Schulungsprogramm_2008.png" alt="" width="620" height="400" /></p>
.
Alternatively you can try cropping the smaller image to the same ratio as the full size image.
