ZF-12300: ZendAmf Resend request and return null
In most cases the zendamf is ok. But when the php executed a long time, the zendamf was getting a trouble.
I looked the running sequence of zendamf and I saw it was sending a request to zend_gateway.php when the zendamf was launched. But in the failed cases, it resended another request to zend_gateway.php and return null and the original request was pending and it's status in chrome network tab was failed.