Seems to be a typical conversion problem with int32/int64/float. As far as I see it, the image numbers were truncated, e.g. 1612525308027.jpg to 1612525308.jpg, which is hinting at int32 (1,612,525,308). The images that work seems to be cleanly truncated, while the missing images were rounded in some way, e.g. 1612524931999.jpg to 1612524932.jpg.

And guess what? The cleanly truncated image is still there, the given number is the start image of the 3Dhanging thread in 3dcg, which is broken in the post.

https://img.guro.wtf/3dcg/src/1612524931.jpg

Only the link in the post is rounded wrong. I guess there will still be some images missing because the truncation will have produced duplicates.