I don’t remember the details of getting my setup going but I believe I based it all on 4.2.1 Apache configuration in the administration manual for UrBackup.
I assume that is a general “I can’t connect to the proxied service” message from Apache so the problem will be some detail of the cgi reverse proxy installation. There may be more clues available in Apache logs.
You will need the required Apache modules correctly installed with their prerequisites which caused me some confusion finding which Debian packages were needed. I can’t help with the CentOS setup steps required for a valid Apache cgi reverse proxy.