I recently created default CPS4.5 and CDS(Citrix desktop server)1.0 and ran into the same problem where I got a user can authenticate error from web interface. The following error is logged in the event log:
servers sent HTTP headers indicating that an error occurred: This message was reported from the XML Service at address [com.citrix.xml.NFuseProtocol.RequestCapabilities] This XML Service could not be contacted and will be temporarily removed from the list of active services. [Log ID: d21d2f79] have xml service share the same port number (port 80) as IIS.
local user group read permission to c:\innetpub\Script\ directory fixed the problem I saw.
interface contacts XML service to authenticate login user and get list of published applications. It looks like the wpnbr.dll (a XML service dll) which resides in this directory is loaded by a user who is not in administrator group.
am not sure if this is a problem specific to my setup. But I installed both products to a clean OS installation. Just want to share my experience with others in case you experience the same problem.
searched Citrix support web site and the closest reference to the same problem is