This would depend on the files they are uploading. I see you have a 450kb filesize limit but say for instance a user uploads a 3mb file and your server by default php would only process a 2MB file so what happens if before PhotoPost even processes anything the file is dropped so there is nothing to process and you would get a message saying all images processed and be sent to the gallery index. That would be my best guess since I have no issue uploading an image.
Not able to replicate anything there. I did upload 6 files and all 6 are there.
Our latest code also is 8.42 so you might be current if your only 8.4. Double check by redownloading the build upload all the new files just incase you did not upload everything last time.