When Location header has port set to 443 in url, i.e.: https://duke.nu:443/kem/ redirect url is set to the same. Reverse-proxies usually don't like the port part in url. In my particulair case it results in a null response from the server and hence a Page object (with nothing in it) i.s.o. an HtmlPage.
Port check inside com.gargoylesoftware.htmlunit.HttpWebConnection line 747:
if (port != 80 && port > 0)
add check for port 443:
if (port != 80 && port != 443 && port > 0)
fixes this issue.
Thanks!
Sander
Fixed in SVN, thanks for reporting.