Had the problem earlier, just solved it in my site a few minutes ago, hope this will also work for you.
1. Go to Settings -> media
2. On the “Store uploads in this folder” enter the default folder “wp-content/uploads”
3. Save
4. Try inserting an image on a post. (this worked for me)
5. For testing sake, I removed the default folder I entered “Store uploads in this folder”, then save.
6. Tried it again on a post, it still works.
