I've got a site that has a plugin that causes problems with the admin-ajax.php file to have 500 internal errors when its turned on. The plugin causes this issue in the ajax file even on pages of the admin that have no changes by the plugin.
Anyhow, first thing you normally try in such a situation is to enable the WP_DEBUG constant to get more details. Bizarrely the 500 errors don't then occur and the site works completely normally including the functionality of the problem plugin.
I'm not getting any help from the plugin developer so I'm on my own with this. Any ideas to get to the bottom of the 500 error as I'd really rather not leave the WP_DEBUG enabled.