Hi
I disabled the varnish cache on the server and it might have had an effect because all images are loading fine in the preview mode now.
However, I got one funny issue which you might have a quick answer for. The original size images are uploaded in 640×427 px, the image preview size is set to max width: 1000px and max height: 800px but some gallery previews show an image of 380×253 and some are shown at the original size (which they all should). The problem persists for complete galleries and not only for single images within a gallery.
Is there a way to tell the image.php file to always use the original size in preview mode?
Thanks again for all your help.