How to hide behind an Apache web server a second one

I have an Apache server configured to server download request for JNLP content, what is used wit OpenWebStart. This content is reachable at http://**second_apache**/appstarter/start/LAMBDA

Now, I want to hide this second_apache behind a central_apache Apache web server, what is already used as reverse proxy for a couple of application servers. But the problem is that this central Apache responds with 404 when I try to download the JNLP.

The request http://central_apache/client is always translated into http://central_apache/weblogin/login?webapp=/appstarter.

The configuration of the central Apache:

<IfModule mod_ssl.c>
  <VirtualHost _default_:443>
    ProxyRequests off
    ProxyVia off
    ProxyPreserveHost on
    AllowEncodedSlashes on
    RewriteEngine on
    RequestHeader set X-Forwarded-Proto "https"
    RewriteCond %{REMOTE_ADDR}        !^1\.1\.5
    RewriteRule .*         -                             [E=R_A:%1]
    RewriteRule ^/jenkins$ https://%{HTTP_HOST}/jenkins/ [L,R=permanent]
    RewriteRule ^/nexus$   https://%{HTTP_HOST}/nexus/   [L,R=permanent]
    RewriteRule ^/sonar$   https://%{HTTP_HOST}/sonar/   [L,R=permanent]
    RewriteRule ^/client$  https://%{HTTP_HOST}/client/  [L,R=permanent]
    ProxyPass        /jenkins http://jenkins:8080/jenkins nocanon
    ProxyPassReverse /jenkins http://jenkins:8080/jenkins
    ProxyPass        /nexus   http://nexus:8081/nexus nocanon
    ProxyPassReverse /nexus   http://nexus:8081/nexus
    ProxyPass        /sonar   http://sonar:9000/sonar nocanon
    ProxyPassReverse /sonar   http://sonar:9000/sonar
    ProxyPass        /client  http://second_apache/appstarter/start/LAMBDA nocanon
    ProxyPassReverse /client  http://second_apache/appstarter/start/LAMBDA
    RequestHeader set X-Forwarded-For   "%{R_A}e"
    RequestHeader set X-Forwarded-Proto "https"
    RequestHeader set X-Forwarded-Port  "443"
  </VirtualHost>
</IfModule>