Now, the check performs a socket connection from a php script to the same server. If that connection fails, you will see the error. There are configurations and security sets on the server that might prevent such conections, like the use of selinux. You can try disabling selinux and see if it makes a difference:

setenforce 0
service httpd restart

Then try again... If it works , then you know that selinux is dennying the connection to make that check. There is a question in the FAQ telling you how to allow those via selinux configs, or you can also disable selinux altogether, is up to you.