What steps will reproduce the problem?
1.run webmin (www.webmin.com/) through a proxy
2. try to connect though the proxy (works)
3. connect to webmin
4. log in
5. <failure>

What is the expected output? What do you see instead?
i expect to see standard webmin, in stead i see

->
Error - No cookies

Your browser does not support cookies, which are required for this web server 
to work in session authentication mode
<-

What version of the product are you using (please check X-Mod-Pagespeed
header)?
X-Mod-Pagespeed: 1.0.22.7-2005
On what operating system?
scientific linux 6.3

Which version of Apache?
Server: Apache/2.2.15 (Scientific Linux)
Which MPM?
prefork 
URL of broken page:
webmin.baziel.com unfortunately i cannot grant you access at this time, but 
webmin is fairly standard stuff. this is my proxy info
<VirtualHost *:80>
        ServerName webmin.baziel.com
        ProxyPass / http://server1:10000/
        ProxyPassReverse / http://server1:10000/
        <Proxy *>
                Order deny,allow
                Allow from all
                AuthType Basic
                AuthName "Password Required"
                AuthUserFile /etc/httpd/password.file
                Require valid-user
        </Proxy>
        ErrorLog logs/webmin1-error_log
        CustomLog logs/webmin1-access_log common
        <IfModule mod_security2.c>
                SecRuleEngine On
        </IfModule>
</VirtualHost>