This error came up for me the second time; the first I was uploading one by one, now I was trying to batch upload.
It uploaded some, then got stuck on one at 100%, and never continued; I got the same result after several attempts.
I tried uploading the pictures one by one; it worked for some, not for others.

I tried uploading the same image after a few hours, and it worked for me.

I was able to upload two images just now, and since then it abruptly started this same error; for me it's always stuck on 100%. Also Chrome.