I have the same problem. I’m using wprobot to add posts to my websites. I tried changing the permissions to 777 and then 755 but it hasn’t fixed the problem. This problem started on January first. Also, my wordpress can’t update the plugins. As I see, the problem is on the months. When the new month comes, wordpress should create new directory named “02”. I will try to fix this issue because it is really annoying. If you can find a fix please let me know.
