I think the issue here sounds like the externally hosted intranet site is giving an externally operational link to internal clients.
Is this link resolvable internally? Can you add a proxy exception for it?
As far as the article goes, it doesn't mention the ability to enable the "append_domain" option, in which case you'll either have to see if the above workaround resolves the issue (i.e. telling clients to go direct to the internal webserver using it's external name), or log a support case with Symantec to investigate.