You probably already have define('WP_DEBUG',false); in wp-config.php. Just change it to true, but it doesn’t matter where it the file that goes, really, so long as it is inside the php tags. It will cause debugging messages to show up, which will hopefully tell you what function is failing. It won’t be pretty– basically just a raw text dump. You can also set WP_DEBUG_LOG and WP_DEBUG_DISPLAY, if you want.