This is not a theme bug. It's an issue with your host.


Usually, you need to get your host to increase the “max_input_vars” to a much bigger number. It’s a common enough issue on hosts not optimized for WordPress (Which would be a red flag for me in terms of long term suitability of your host for WordPress).

See here for more details:


https://wordpress.org/support/topic/menu-size-limit

http://www.wpbeginner.com/wp-tutorials/how-to-fix-the-custom-menu-items-limit-in-wordpress/

http://sevenspark.com/wordpress/menu-item-limit