Outlook 2003: "cannot read the calendar. There is not enough memory to perform this action"

One of our customers has reported this message appears when accessing a shared calendar on SBS 2008 ( Exchange Sever 2007)

At first i thought it may be a calendar issue but if i log in as the user whose calendar it is there seems to be no problem. Could this be that the users that are seeing this message have too many profiles within Outlook ???

Any info or pointers on this would be greatly appreciated..

Thanks in Advance

Mike