If this is indeed happening then reinstalling will not help, as it is not a bad setting or a bad installation which would be the cause of this...
MerryChristmas, is it always the same contact for which this happens? Or doesn't it matter which contact it is? (although I think this shouldn't matter since Plus! will search for windows owned by Messenger (the current process) I think, no matter what their captions are, not sure though)
PS: MerryChristmas, Merry Christmas too