apache Proxy returns random 403

Question

I'm using 2 VMs configured with Apache2 as a Proxy for the Tomcat server, but I have a very strange behavior I have sometimes HTTP 403 error on my front end, I tried different configurations but I can see that depending on the requests coming from my Load Balancer (plugged with my 2 VMs Apache) it returns HTTP 403 randomly (and not for the same requests as well).

Here is the flow:

Front end -> Load Balancer (443)

• -> Apache2 Reverse Proxy (443) -> Tomcat (8080)
• -> Apache2 Reverse Proxy (443) -> Tomcat (8080)

I run this Apache2 Reverse Proxy on Ubuntu, here is the configuration for my Apache2 Proxies:

<IfModule mod_ssl.c>
    <VirtualHost *:443>
            ServerAdmin webmaster@localhost

            ErrorLog ${APACHE_LOG_DIR}/error.log
            CustomLog ${APACHE_LOG_DIR}/access.log combined

            <Proxy *>
                    AddDefaultCharset Off
                    Order deny,allow
                    Allow from all
            </Proxy>

            Header add Access-Control-Allow-Origin "https://myfrontend.com"

            SSLEngine on

            SSLCertificateFile /etc/ssl/myfrontend.key
            SSLCertificateKeyFile /etc/ssl/privatemyfrontend.key
            SSLCertificateChainFile /etc/ssl/ca_bundle

            SSLProxyEngine on
            ProxyRequests off

            SSLProxyVerify require
            SSLProxyVerifyDepth 1
            SSLProxyCACertificateFile "/etc/ssl/myfrontend.key"
            SSLProxyCheckPeerName on

            ProxyPreserveHost On
            ProxyPass / http://localhost:8080/back/
            ProxyPassReverse / http://localhost:8080/back/

    </VirtualHost>

any idea why I'm getting such 403 errors?