Yet another gtalk "not authorized"

[B Wooster]

Have tied all of the help pages out there, with different
ports/servers from each page, but a non-gmail address always fails on
Vista with "Not Authorized".

Google Talk client on Vista works.
Pidgin 2.6.6 on Linux works.
Pidgin 2.7.3 on Vista fails.

Only difference I can see in the debug log is this:
(10:54:48) sasl: Mechs found: PLAIN X-GOOGLE-TOKEN
(10:54:48) jabber: Sending (ssl) (someone at example.com/): <auth
xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='PLAIN'>password
removed</auth>
(10:54:49) jabber: Recv (ssl)(77): <failure
xmlns="urn:ietf:params:xml:ns:xmpp-sasl"><not-authorized/></failure>

The send is missing  xmlns:ga - could that be a problem?

On the working Pidgin (older 2.6.6):
(10:58:40) jabber: Sending (ssl) (someone at example.com/): <auth
xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='PLAIN'
xmlns:ga='http://www.google.com/talk/protocol/auth'
ga:client-uses-full-bind-result='true'>password removed</auth>
(10:58:40) jabber: Recv (ssl)(51): <success
xmlns="urn:ietf:params:xml:ns:xmpp-sasl"/>

Anything else I should try?