If you ever had problems with the WordPress files ownership or permisions this might help you.
In my server I had different users and groups. I couldn't get the auto-upgrade (core, themes and plugins) to work.
I was being lazy about it. You know, it's one of that things that you keep avoiding until you stumble across the solution. So, all I had to do is change the ownership of the WordPress files to www-data.
Use the following command:
chown -R www-data:www-data /my-wordpress/folder
In some cases you might also need to change the file permision which can be done with the following commands:
chmod 777 /my-wordpress/folder
For security reasons you should restore the right permissions once you're done with the changes:
chmod 775 /my-wordpress/folder
I just found I can't surf my WordPress directory using FTP. I'm guessing it's caused because my FTP password doesn't have the right permissions since we just changed them to www-data.
I'm still looking at this, any ideas?