I just had this issue. And none of the suggested solutions I found worked.
For me it ended up being quick and stupid.
1) Check the folder via ftp client, see if it has any content (mine did not)
2) Delete it
3) Try to upload via WP media uploader again
4) That worked for me
I’m pretty sure this won’t be the solution for most people, but it worked for me, so you never know. My only assumption can be the folder was written incorrectly on creation when I attempted to upload my first media file for that month.
Good luck!