I am also experiencing this bug, which only started occuring last night. stderr gets "QGArray::find: Index 0 out of range" while a dialog appears: "Connection Lost - MSN Plugin" (with the obvious contents).
GAIM still functions perfectly, so it's a bug with Kopete; like I say, it only started occuring yesterday. I managed to log in once, and Kopete had lost my contact grouping (just in case that helps track this down).
Edit:
http://bugs.kde.org/show_bug.cgi?id=76520 has all the answers.[/url]