Strange messenger problem

Need some help....

I've got at really strange problem wih my messenger.

The problem only appears between myself and one of my contacts. When I try
to send him a message, the message appears on his screen. When he's replying
to me, I cannot see what he writes to me. Then, after a couple of seconds,
my msn-messenger logs out!

This only doesn't occur if I sign in on another computer.