I'm facing the same issue, using an internal QVS used also for custom user authentication. Then installing a DMZ Web server in another domain.
I can set up the config.xml file so that it access to the internal QVS server but web server cannot contact the QV Server.
I guess this is because the services account are not recognized in either side ...
Did you get any update or did you solve this issue ?
Thanks for sharing.
When I sent this post, there was an issue with "Admin" communication between two servers if they were not in the same domain.
This problem was present in V9.
I guess it was solved on the V10 by specifying an account between QVWS and QVS across domains. This must be confirmed by the support team of QlikView.
As a workaround we installed a reverse proxy in order to only allow /qv*on port 80 to come in the QVWS server.
I hope this will help you.