I'm aware admin-ajax.php is intended to return '0' if there's no action set. My issue seems to be that it's returning an error after the '0'.
Accessing the file mu.site.com/wp-admin/admn-ajax.php directly returns '0' as expected.
Accessing the file mu.site.com/sub-site/wp-admin/admn-ajax.php directly returns '0 OK The server encountered an internal error or misconfiguration and was unable to complete your request. [ check your error logs]"
Naturally, my error logs have no information shown even though all sources say that error logging is enabled.
Possibly it's due to me having a subdirectory multi-site set up on a subdomain?
I first noticed the issue because I tried to upload a file through the Media Library. The file gets uploaded to the server (verified via FTP), but I receive a generic "HTTP error" error in the Media Library and no files are added to the WP database. I've now noticed admin-ajax fails throughout the WP Admin. The error above also shows in the Network tab of Chrome's developer tools when uploading.
Everything is working as expected on my primary site, this just occurs on my newly created sub-site. I did just convert a single site WP install to multisite so the sub-site has no other content / is brand new.
Disabling plugins & using a standard theme has no impact.
Setup is on Network Solution's "Web Hosting for Essential (Hosting-Unix)" platform. Using WordPress 4.7.3.
I'd really appreciate any ideas you have!
