How to know it: Open the wp-config.php file and change for this line:
define('WP_DEBUG', false);
to true
define('WP_DEBUG', true);
Problem: Same situation like this using WPML Plugin.
Solution: check database, there are some rows which id is zero. Try to delete all of them.
Then, the table must not have primary key, so set a primary key and set auto increment.
