error proxying from virtual server to internal server


I’ve setup a new virtual server so it proxies the request to one of our internal server, which work in https and point to it’s internal IP address. I’ve added the “SSLProxyEngine on” “SSLProxyCheckPeerCN off” “SSLProxyCheckPeerName off” lines to the site config. Now when I try to access the https external URL, I get the normal login screen of our internal server, but with elements missing. If I try to login, I get :
Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request POST /core/user_settings/user_dashboard.php.

Reason: DNS lookup failure for:

In the Apache error log, I have a lot of lines like these:

DNS lookup failure for: returned by /themes/enhanced/images/icon_pause.png, referer:

Where should I check for the source of these errors?

thank you!