I had several problems with this issue but i finally found a solution for the problem. The key is to add “*.pad.domainname” to the ServerAlias direcive.
Here is my solution for running an Etherpad Pro.
<VirtualHost *:80>
ServerName pad.braegelmann.com
ServerAlias *.pad.braegelmann.com
Options ExecCgi Includes MultiViews Indexes FollowSymlinks
ErrorLog /var/log/apache2/etherpad-error.log
TransferLog /var/log/apache2/etherpad-access.log
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
Alias /sitemap.xml /ep/tag/\?format=sitemap
ProxyPreserveHost On
ProxyPass / http://localhost:9000/
</VirtualHost>