Unable to resolve the URL to a valid IP address when setting up DirectAccess

I'm setting up a DirectAccess server, and running through the setup wizard.  On step 3, it's asking for the url of the Network Location Server.  I give ithttps://webservername.domainname.local/directaccess and click Validate, and I get an error saying "Unable to resolve the URL to a valid IP address".  nslookup is able to resolve the name of the webserver OK.  If I open a browser on the DirectAccess server, and typehttps://webservername.domainname.local/directaccess in the address bar, it opens default.htm on the website, so that's working fine.  The webserver is on a perimeter network, so is on a different IPv4 subnet to the DirectAccess server.  Does this make a difference?  Does the URL need to resolve to an IPv6 address?

Thanks.