带有ProxyPass的Apache httpd错误“代理：ap_get_scoreboard_lb”

我设置了apache来代理tomcat，但是当我定位页面时却遇到了以下错误。有时会出现空白页或503：

[错误]

[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/

我在虚拟机上配置了两个虚拟主机，如下所示：

[http主机]

<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虚拟主机]

<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>

我的系统详细信息是：Apache / 2.2.22（Ubuntu）mod_jk / 1.2.32 mod_ssl / 2.2.22 OpenSSL / 1.0.1 mod proxy_http也已启用。

似乎在停止然后启动apache和apachectl graceful重新启动（即实际上停止并启动而不是apachectl restart）之间存在区别，如在该站点上发现的：

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.