I'm getting the same error trying to upload a level. I'm assuming it's saying some sort of file is missing, but I'm not sure what file could potentially be missing.

EDIT: Check to make sure you're copying the folder's entire path correctly. I found a spelling mistake when I was uploading mine with a picture. Specifically, I copied the path right, but the picture had a different filename.

I have it fixed now, I put the image file in directly, and not the folder containing it.

Glad to hear you got it working! The more workshop stuff, the better in my opinion.