After inspecting the requests with the Firebug Console, I saw that the culprit was the request:
I tried the normal fixes such as checking permissions, updating everything, and even reinstalling WordPress but to no avail. Then I started looking on Google for a potential fix and found this article which not only provides the solution, but also a detailed explanation of why the error occurs in the first place.
As soon as I made the recommended updates, all the sites started working properly with no issue whatsoever.
Full credit goes to the article author. Just posting here for my own reference more than anything else!
UPDATE: As the original article no longer exists I’ve posted the instructions below:
It turns out the solution to this is quite simple. Nginx and php-fpm need to run as the same user and the directory
/var/lib/nginx needs to be owned by that user. Personally I like to run all my web-related services under a
www-data user, but running it under a
apache user is also common. To change the user nginx runs under, edit the main nginx config file usually located at
user www-data; worker_processes 4; pid runnginx.pid;
Now to change the user php-fpm runs under we need to edit its config file. On my Amazon Linux system this was located at
/etc/php-fpm.d/www.conf while on my old Debian setup it was located at
/etc/php5/fpm/pool.d/www.conf. There are two user-related settings in the config file. First, if php-fpm runs as a socket (as opposed to listening on a port), the user and group owner of the socket should be set to the same user as nginx so that nginx can access it to run php files. This is not relevant for our error, but can cause other issues if nginx doesnt have permissions to access the socket. Second, the user and group that the php-fpm process runs under needs to be the same as nginx.
user = www-data group = www-data listen = varrunphp5-fpm.sock listen.owner = www-data listen.group = www-dataL
To change the ownership of the nginx lib directory simply run the command
chown -R www-data:www-data /var/lib/nginx as root (
sudo will do on most systems). Finally, if we edited any config files, we need to restart the appropriate services for the changes to take effect. On most systems this can be done by running
service nginx restart or
/etc/init.d/nginx restart as root. For php-fpm the service name is