If you ever copy the Iteration Backlog.xlsx file from one Team Project to another (usually because you are upgrading an old Team Project to the TFS 2010 templates), then you probably get one of these errors:
“T80076: The data in the work item is no valid or you do not have permissions to modify the data. Please correct the problem and retry.”
“TFS208103: The initialization of the workbook to connect to Team Foundation Server was not successful….”
You need to reconnect the spreadsheet to the right place.
There’s instructions on how to do that: http://msdn.microsoft.com/en-us/library/ff452586.aspx (almost all the way down), look for “To bind the Iteration Backlog workbook to the Iteration Backlog query”.