I have tried to narrow down the cases when this happens, for example when an image is large (15MB), average (7MB) or small. Strangely, it does not always happen with large images. It seems entirely random:
Doubleclicking on an image first shows the thumbnail (for less than a second) while the preview loads. But in some cases I am presented with this, even though I ran the thumbnails php script.
The current workaround is to hit the + button to zoom, since that loads the full image, often slightly zoomed, forcing me to have to zoom out.
Filing as bug because I could not figure out why this happens, the next image loads its preview just fine, then 3 don't, then a few do again etc.
Customer support service by UserEcho