Right now, MSN is in the process of doing away with the msnusers url and associated servers. In the past, this was where all group documents folders, pictures and message attachments were stored. As you can imagine, this is a rather large undertaking and they are doing it in segments (most likely, a server at a time).
Everytime MSN undertakes something of this magnitude, groups will be affected. Right now, the groups on the server where the transition is being made are being affected, while other groups are fine.