Offer Remote Assistance Probs

Hi all,
I am running a SBS2003 R2 SP2 domain, with the new RDP client on the server
and all workstations (today I even installed the new release of an update
for this client, V2, see KB925876). The feature "Offer Remote Assistance"
works fine from all VISTA clients to the server or to an XP client. I've
tried so many things now, but I cannot get it to work from an XP client to
the server or vise versa. Could it be an issue with the new RDP client?
Apparently it's working fine with the new VISTA, but from what I know this
is a completely different application than on XP or 2003.
First everything seems fine, I type in the computer I want to connect to,
but then it says (even though there is only ONE user logged on th target
computer:
Several users are logged onto "xp1". Select the user you want to assist.
Still, I select the only possible logged on user, but then:
A Remote Assistance connection could not be established. You may want to
check for network issues or determine if the invitation expired or was
cancelled by the person who sent it. See the connection issues help topic on
the Remote Assistance invitation status page for additional troubleshooting
information.
As I said, I've tried quite a bit already, firewall, group policy settings,
HelpAssistant User Accounts and so forth but no success. The fact that it
all works fine from a VISTA client implicates to me that the basic settings
are correct.
What am I doing wrong here?
Thanks,
Rene