I ran into this problem when trying to enable proxy function on apache http:
My httpd.conf

LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_connect_module modules/mod_proxy_connect.so
LoadModule proxy_ftp_module modules/mod_proxy_ftp.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule proxy_fcgi_module modules/mod_proxy_fcgi.so
LoadModule proxy_scgi_module modules/mod_proxy_scgi.so
LoadModule proxy_ajp_module modules/mod_proxy_ajp.so
LoadModule proxy_balancer_module modules/mod_proxy_balancer.so

<VirtualHost localhost:57173>
ServerName localhost
<Location /balancer-manager>
SetHandler balancer-manager
Order Deny,Allow
Deny from all
Allow from localhost
</Location>
<Proxy balancer://mycluster>
BalancerMember http://localhost:9999
BalancerMember http://localhost:9998 status=+H
</Proxy>
<Proxy *>
Order Allow,Deny
Allow From All
</Proxy>
ProxyPreserveHost On
ProxyPass /balancer-manager !
ProxyPass / balancer://mycluster/
ProxyPassReverse / http://localhost:9999/
ProxyPassReverse / http://localhost:9998/
</VirtualHost>

When I check the log:

[Sat Oct 27 00:16:24.506220 2012] [proxy:crit] [pid 3153] (13)Permission denied: AH00920: Failed to reopen mutex balancer://mycluster in child
[Sat Oct 27 00:16:24.506407 2012] [proxy_balancer:crit] [pid 3152] (13)Permission denied: AH01206: Failed to init balancer balancer://mycluster in child

Can someone please help me :( I tri googling it but there was no answer for this problem.
Thank you very much