Apache service isn't starting in a FreePBX HA Cluster.
This is normally caused by two things:
Dependency Failed to Start
Apache relies on 'httpd_ip', 'httpd_fs', and 'floating_ip'. If any of those dependencies are unable to start, the Apache service will also not start.
The *_ip services will fail to start if the machine is unable to determine which interface they should run on, or, if another machine already has that IP address on another network segment.
If the httpd_fs service is unable to start, this is often because of a corruption on the DRBD block device, or because DRBD isn't running at all.
Apache 'status' page is being blocked
FreePBX HA enables the '/server-status' module, and restricts it to only being accessible from 'localhost'. It's possible that an admin may inadvertently tighten the security of the machine, thinking that this may be 'leaking' information. However, it is a requirement for the cluster manager to be able to query apache for its health status. Remove any changes you have made.