Meta Box uses nonces to prevent misuse or malicious requests. As the nonce fields are outputted directly in the HTML, they might be cached by caching plugins such as W3 Total Cache, WP Super Cache, etc. And thus, the verification of the nonce might not work properly and break Ajax's request. In this case, please do not cache the page where the form is embedded (both caching plugins allow you to do that). For more information, please read this technical article.