Shared Workbook on domain cannot be opened by another user when one person has it open- error states file cannot be found

We just went to Windows 7 and are using Office 2010.  The workbook is set to shared and to allow multiple people to edit.  When one person who has rights to edit it opens it, and another user who has edit rights try to open it, they receive a message that the file cannot be found.

When we log into a virtual server with Windows XP and Office 2007, the same excel file can be accessed by multiple users.

This is happening with any shared excel 2010 file we checked.  What is causing this?

We are controlling access to the files with AD groups.