This happened to me a couple days ago, with symptoms similar to described above, so just providing a supporting data point: tried to upload 6 pictures at the same time, the first one went to 100%, at some point the 6th one disappeared entirely and then the entire window seemed stuck and had to force-reload the page to recover. After trying a couple more times with the same behavior and noticing the pattern, I uploaded the first 5 by themselves, then the 6th one and that worked, fortunately.

All 6 basically the same size/dimensions; Chrome browser on a Chromebook.