Have you tried to do it manually with steamCMD, it just seems to work a lot better then the inbuilt uploader.
If you are using windows, don’t load it by double clicking, use command prompt and navigate to it, that way you can copy your path to the .vdf files with windows explorer, then right click paste (or just right click with win10 or newer versions of command prompt)
login [username] [password]
workshop_build_item [path to file]\Modname.vdf
exit
exit
done.