To find out where the issue really is you will need to use your host’s admin interface to edit your wp-config.php file in your site’s wp root directory and change the WP_DEBUG to true. This will print out message where the problem really is.
You can search the web if you need help for changing that value. You’ll probably want to change back to false after you figure out the issue.
This plugin says it enables WP_DEBUG when it is activated, then sets back to false when deactivated.