Cannot sign in Messenger XP in compatibility mode

I'm running Windows Messenger 4.7 on XP.
I set it to compatibility mode (Windows 2000) and it used to work.
But since a week (or so) ago I cannot sign in:
When I start Messenger I get a "Click here to sign in" link, I click
on it, give the name and password. The link with "Click here to sign
in" comes back and I'm not signed in.
When I deselect runing in compatibility mode everything works fine and
I'm able to sign in.
However, I need to run it in compatibility mode for other reasons.

So, what can I do to be able to sign in in compatibility mode?

Thank you.