I’m pretty sure it’s not related to preloading the images per se (or only indirectly) as the error messages you get suggest that the script sometimes fails to build your page’s structure
After reloading your experiment a few times, it did fail to preload the image 1.png on occasion—I think that what happens is that when your S3 bucket receives several requests for the ZIP file from the same address within a short delay, it can put those requests on hold to prevent too much traffic, and the default preloading time is not long enough for the zip files to be downloaded when those delays happen (I see you’ve been playing around with CheckPreloaded though, that’s probably a good idea since you can make the max preloading time longer)
But again, when that happens, the trial loads fine in my browser, I just can’t see one of the two pictures, but I can still see the other one and I can press F or J to validate the trial. What browser do you use? Could it be Safari by any chance?
Jeremy