It is a known issue with MSN Messenger 7.5 and Messenger Plus! 3.54.132.
In a couple of days a new version of MsgPlus! will be released that will be 100% compatible with MSN Messenger 7.5.
For now, you can disable Custom Names. Or downgrade to MSN Messenger 7.0.
If you use the StuffPlug plug-in, upgrade to the latest version (it's a beta).