Hi Alex
Thank you for your reply.
I followed the instructions and cleared all caches. Unfortunately the problem persists.
With the square image, the large thumbnail had a size of 1030x1030px. With the rectangular image the large thumbnail has a size of 1030x687px. If I check the post in the backend, I notice that the src-address of the image still points to the thumbnail with the size of the old image (1030x1030px). Therefore I’m not surprised that the old image is still displayed in the frontend.
Thanks