[ClusterLabs] The proxy server received an invalid response from an upstream server.

[Jason Long]

Hello,
I want to launch a Clustering for my Apache Web Server. I have three servers:

1- Main server that acts as a Reverse Proxy
2- The secondary server that when my main server stopped, work as a Reverse Proxy
3- Apache Web Server

The client ---> Reverse Proxy Server ---> Apache Web Server

The IP addresses are:
Main Server (node1)                    : 192.168.1.3 
Secondary Server (node2)           : 192.168.1.10 
Apache Web Server (node3)        : 192.168.1.4

On the main and secondary servers, I installed and configured Apache as a Reverse Proxy Server. I created a Virtual Host and my Reverse Configuration is:

<VirtualHost *:80>
    ProxyPreserveHost On
    ProxyPass / http://192.168.1.4/
    ProxyPassReverse / http://192.168.1.4/
</VirtualHost>

As you see, it forward all requests to the Apache Web Server.

I installed and configured Corosync and Pacemaker as below:

On the main and secondary servers, I opened "/etc/hosts" files and added my servers IP addresses and host names:

192.168.1.3    node1
192.168.1.10  node2

Then installed Pacemaker, Corosync, and Pcs packages on both servers and started its service:

$ sudo yum install corosync pacemaker pcs
$ sudo systemctl enable pcsd
$ sudo systemctl start pcsd
$ sudo systemctl status pcsd

Then Configured the firewall on both servers as below:

$ sudo firewall-cmd --permanent --add-service=http
$ sudo firewall-cmd --permanent --add-service=high-availability        
$ sudo firewall-cmd --reload

After it, on both servers, I created a password for the "hacluster" user, then on the main server:

$ sudo pcs host auth node1 node2 -u hacluster -p password
node1: Authorized
node2: Authorized

Then:
$ sudo pcs cluster setup mycluster node1 node2 --start --enable
$ sudo pcs cluster enable --all
node1: Cluster Enabled
node2: Cluster Enabled

After it:
$ sudo pcs cluster start --all
node1: Starting Cluster...
node2: Starting Cluster...

I checked my clusters with below command and they are up and running:
$ sudo pcs status
...
Node List:
  * Online: [ node1 node2 ]
....

And finally, I tried to add a resource:
$ sudo pcs resource create floating_ip ocf:heartbeat:IPaddr2 ip=192.168.1.4 cidr_netmask=24 op monitor interval=5s
$ sudo pcs resource create http_server ocf:heartbeat:apache configfile="/etc/httpd/conf.d/VirtualHost.conf" op monitor timeout="5s" interval="5s"

On both servers (Main and Secondary), pcsd service is enabled, but when I want to see my Apache Web Server then it show me below error:

Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request
Reason: Error reading from remote server

Why? Which parts of my configuration is wrong?
The output of "sudo pcs status" command is: https://paste.ubuntu.com/p/V9KvHKwKtC/

Thank you.