(Answered by the OP in a question edit. Converted to a community wiki answer. See Question with no answers, but issue solved in the comments (or extended in chat) )
The OP wrote:
The answer is here: http://sysadminsjourney.com/content/2010/02/01/apache-modproxy-error-13permission-denied-error-rhel/
Which is a link to a blog that explains:
SELinux on RHEL/CentOS by default ships so that httpd processes cannot initiate outbound connections, which is just what mod_proxy attempts to do.
If this is the problem, it can be solved by running:
/usr/sbin/setsebool -P httpd_can_network_connect 1
And for a more definitive source of information, see https://wiki.apache.org/httpd/13PermissionDenied