This is a bug in Excel that sometimes happens when selecting multiple items on a dimension, where the dimension values selected aren't reflected in the other pivot hidden table. The suggested work around is to delete the tab in the spreadsheet and copy another one and try it again. If that fails, try creating a whole new spreadsheet by coping a previous one that was working. If the problem persists, then please create a support request at www.blackbaud.com/support, noting the name of the spreadsheet and details to reproduce the error, and referencing this solution.

See What are the steps for OLAP Cube Troubleshooting? for more information.