Hi guys
We had the very same problem with one of our blogs (WP 3.8.1). But by incident we checked:
Settings > Media
Usually it’s a very short page ending with “Uploading Files” and a checkbox saying “Organize my uploads into month- and year-based folders”, and then the “Save Changes”-button.
But in THIS window was below a suggestion to a path to upload files to. And, mind you a path, saying something completely bullocks. We were allowed to change this, and WP even suggested:
wp-content/uploads
We changed and saved and… VOILá!!! Now we can upload files again!
Hope it works for you guys too ??
/Ken