The problem was caused by Smart Product Quantity plugin. I’ve disabled the plugin, clear cache and check now. Contact support of Smart Product Quantity to get assistance.
Please note that we do not give a guaranty for full theme compatibility with all existing 3rd party plugins. Sometimes, not always, you need additionally customize the theme or plugin to make them work together. Unfortunately, it’s not possible for the author to make the theme compatible with all the existing plugins and additional customization is outside the scope of our support.