If you can find away to get it out of the xml it will work.
I don't know why this is happening but it is definitely your problem.
Looking to your topic because I've had the same issue on a customer site, I have searched a little.
I've seen on the UCCX admin web page that the site is trusted by IE. So I've trusted my web server in IE. And it seems to work ...
In the other hand, dev team of the web server seem to have problems with past certificates ...
Moreover, it works fine in HTTPS but not in HTTP. I think that it could be a difference behavior of the web server between http and https.
A way to workaround this issue is to use the "Set Xml_Doc = URL[URL_var]" in place of Get URL Document.
For information, I use UCCX 5.0.2 on a Win2K3 SP1.
Hop this help ...