Sources - Excel Workbooks
Created a .twbx in Tableau 8 and published.
Opened the same .twbx in Tableau 10.0.02. Opened well. Live Connection.
Attempted to create cross DB. "internal error - an unexpected error occurred and the operation could not completed"
Tried to create cross DB join in a new .twbx, this worked. The above issue is specific to the workbook.
This is the type of issue that should go to Tableau Support as it looks like you may be experiencing a defect. They may need logs and a screenshot of any errors.