Apache httpd error "proxy: ap_get_scoreboard_lb" with ProxyPass

Question

I set up apache to proxy for tomcat but I am getting the following error when I target the page. I sometimes get a blank page or a 503:

[Error]

[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/

I have two vhosts configured on the vm as follows:

[http host]

<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>

My system details are: Apache/2.2.22 (Ubuntu) mod_jk/1.2.32 mod_ssl/2.2.22 OpenSSL/1.0.1 mod proxy_http is also enabled.

Solution

It seems there's a difference between stopping and then starting apache and doing a apachectl graceful restarting (i.e. actually stopping and starting as opposed to apachectl restart) as found at this site:

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.

Comments

[nikc.org]

FWIW, apachectl restart isn't enough either; I had to do an apachectl stop; apachectl start to prevent this error from occurring (also with Apache httpd 2.2.2).

[Dark Star1]

@NicholasRiley by Yeah That's what I meant by restarting. I perhaps should have been clearer on that point.

[Anthony Geoghegan]

Thanks for this. I was getting the same errors when I was changing a forward proxy configuration and then restarting using graceful.