(Update: Edited the problem description for clarity.)
From her review of the Messenger 7 Beta I know that Abbie uses Messenger Plus.
Messenger Plus is usually the culprit when this error shows up. Messenger Plus allows users to add characters, in one form or another, to their display names that Messenger doesn’t handle.
When you close the conversation window in Messenger, Messenger updates the history file. During the update to the history file Messenger encounter the character that is outside the range of supported characters and this error is the result.
So, how do you fix this?