Its been a while since I uploaded, but I remember some of the issues I had with it. I remember having to disable the Steam Guard before I uploaded. It might also be caused by special symbols in the title or something like that, so be sure to get rid of that.
I followed this video exactly and it ended up working:
I would suggest you change visibility to public at first, then after it successfully uploaded set it to hidden by accessing it directly through Steam. I have had problems with private visibility in the past when making mods for other games. Worth a try.