[WP 4.9.4] Can’t edit PHP files

Having this plugin active prevents me from editing .php files using the theme editor.

Just php files. Everything else (e.g. css) is fine.

What happens is that, when I click “Update File”, it would hang for exactly 60 seconds, then this message would appear:

Something went wrong. Your change may not have been saved. Please try again. There is also a chance that you may need to manually fix and upload the file over FTP.

I’ve already ruled out security plugins (Wordfence). I also double-checked file permissions. I’ve isolated the issue to this plugin.

Using Chrome’s developer tools, I see that the call to admin-ajax.php to save the php file results in a 504.