syndrome5 shared the following useful information:
After the Linux installation, the 503 error appears (on CentOS 7).
To avoid this problem WITHOUT deactivate SELinux :
1. Download the attached file httpdPolicy.zip and unzip it to get the .pp file
2. Launch this commandline : semodule -i httpdPolicy.pp
3. Current mode for SELinux sestatus would be “enforcing” & reboot