mercator on "Stopdesign - Thumbs not showing"

Hi, I am on version Zenphoto 1.4.3.4 [10988] (Official build) and encounter a strange behaviour with the Stopdesign theme: If I remove the thumbs cache, all my galleries show normal - the first time. But when I revisit a gallery, some of the only show empty thumbs (some=some galleries show none, some show al). The Default theme seems to work fine. Any ideas what this could be?
Thanks so much
Mercator