Circle Hover Effect not working right

It looks like the issue is because the images themselves are too short, try changing their heights to 170px.

Thanks for your quick response.
I’ve chjanged the thumnail-size to 170×170, had then the thumbnails recreated. No change. I uploaded a test picture to both the wordpress installations, and it creates all the picture sizes on both of them (150×150, 270×250, 300×225, 1024×500).
But somehow the https://en.lebirne.com site takes the wrong picture. When inspecting the item in the browser the English site has a rectangle picture whereby the German site (https://de.lebirne.com) has a square image

The recommended image is 270x250px.

I used Photoshop to customise(!) my images to that standard size, setting up a transparent or colored 270×250 Blank File and then moving the miscaled image into it and centreing it.

The images should be 270×250. The English site is picking up a very large image (I don’t know why).

Pragmatic answer: delete it from your media library and re-upload.