Seems more like a firewall issue because you authenticate before you open a document, before the Plugin becomes active.
Windows native clients (like the Plugin) use port 4747 to talk to QVS. Firewalls typically block this port. QVWS has a way around this by using so called Tunneling
Are you using IIS as DMZ web server? Then you should enable Tunneling (see QlikView Server Ref, chapter Tunelling using Microsoft IIS) byt performing some additional steps.