Errore Apache httpd "proxy: ap_get_scoreboard_lb" con ProxyPass

Ho impostato apache su proxy per tomcat ma ricevo il seguente errore quando scelgo come target la pagina. A volte ricevo una pagina vuota o un 503:

[Errore]

[Mon Dec 03 04:58:16 2012] [error] proxy: ap_get_scoreboard_lb(2) failed in child 29611 for worker proxy:reverse
[Mon Dec 03 04:58:16 2012] [error] proxy: ap_get_scoreboard_lb(1) failed in child 29611 for worker https://localhost:8443/
[Mon Dec 03 04:58:16 2012] [error] proxy: ap_get_scoreboard_lb(0) failed in child 29611 for worker http://localhost:8080/

Ho due vhosts configurati sul vm come segue:

[host http]

<VirtualHost *:80>
  ServerName www.mysite.net
  ServerAlias mysite.net

  ProxyRequests Off
  ProxyPreserveHost On

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

  ProxyPass / http://localhost:8080/ retry=0
  ProxyPassReverse / http://localhost:8080/ retry=0
</VirtualHost>

[ssl vhost]

<VirtualHost *:443>
        ServerName www.mysite.net
        ServerAlias mysite.net

        ErrorLog /var/log/apache2/error.log

        LogLevel warn

        CustomLog /var/log/apache2/access.log combined
        ServerSignature On

        SSLEngine on
        SSLProxyEngine on
        SSLCertificateFile /etc/apache2/ssl/server.crt
        SSLCertificateKeyFile /etc/apache2/ssl/server.key

        ProxyRequests Off
        ProxyPreserveHost On
        ProxyPass / https://localhost:8443/ retry=0
        ProxyPassReverse / https://localhost:8443/ retry=0

</VirtualHost>

I dettagli del mio sistema sono: Apache / 2.2.22 (Ubuntu) mod_jk / 1.2.32 mod_ssl / 2.2.22 OpenSSL / 1.0.1 mod proxy_http è abilitato.

Sembra che ci sia una differenza tra l'arresto e l'avvio di apache e il apachectl gracefulriavvio (ovvero l'arresto effettivo e l'avvio anziché il riavvio di apachectl ) come riscontrato in questo sito :

This is a bit of a guess, and probably wrong, but does stopping and 
starting the server rather than restarting (they're different, 
restarting (including gracefully) doesn't kill the master process) 
help? As I understand it, there are issues in mod_proxy regarding the 
scoreboard size when changes to the config are made between restarts.