Some of my product images are missing on my category pages. The product has all the images set and they show in the backend and also on the product page itself. Just the category image is missing and the placeholder image is shown.
I tried reindexing, disabling and clearing all caches, nothing helped so far. The images are about 300kb to 400kb in size and the problem also occurs with the default list.phtml. Any ideas what else might be wrong?