Problems with MSN connecting over ISA2004 (SBS2003 Prem.)

Hello,
I’ve got the following Problem:
We’re using Microsoft Small Business Server 2003 Premium Edition which
includes ISA2004 (SP3). Since the Update to ISA2004 SP3 our MSN Messenger
doesn’t connect anymore. Therefore I uninstalled the old Version and
installed the New Windows Live Messenger 8.5 Beta, but the Result was the
same than before: When I make the Test directly under
“Options�-“Connection�-“TCP� then the Messenger is telling me, that he is
able to connect, and I can initiate a Messenger Session, but if I try to
connect the Messenger by using a Messenger-Account I only get this error:
81000306. If I connect the Computer directly to the Internet-Router, then the
Messenger is working correctly and it will connect fast the
Messenger-Service. There fore I’m sure, it’s a Problem with the ISA2004.
I tried to define some Rules which allows MSN Messenger-Traffic for this
Machine, but this doesn’t work. Also it doesn’t work if I allow all Traffic
for the whole LAN.
But if I deactivate the Rule “SBS Protected Networks Access Rule� in ISA,
then the Messenger will connect correctly. But I don’t want to do that,
because it’s a Part of the default-Rule-Set and I’m not sure, what’s going to
be happen, if I’ll do so.
Is there a way to allow MSN / WLM for the Users in our LAN?

Regards,
Mike