so you are able to access the qlikview site by standard http?
Then I would guess it is a firewall issue at customers side or server side and you have to explicitely grant http access. I guess all other IPs are only https allowed.
Could you post the adress you were able to access it from other IPs?
I just tried http://reports.ddms.nl/qlikview/index.html but got only a timeout, the link above worked (no credentials though, of course, to log in).
But just guessing,
Is the customer using a proxy to get access to external web sites? As port 10443 is not a standard port for HTTPS (standard is 443), my guess is that they won't allow it through the proxy.
Either configure the site to use default port 443 for HTTPS or configure the proxy to support the above site (including port 10443)