Please provide the details on how the calendar was shared. Was delegation used or permissions on the calendar itself? When you say shared, is user 1 trying to put user 2’s calendar on their folder list or just view it in their own calendar? Check the 2007 user’s permissions and make sure they did not grant permissions that the 2003 user will not be able to see. Select just the permission level and nothing else. One user of 2007 granted rights the 2003 user could not see in our organization and the 2003 user only saw a blank calendar with nothing on it until this was changed. More details are needed. Thanks!