The root cause for this problem has not been identified.
However, this error dialog message pops up when the add-in can not export the query cause the object to list the content is already in-use. This problem always happens after a query was already run via excel export.
Please perform these steps in this order and it should allow you to continue. Please run your export query after each step.
Continue to the next suggested step only if unsuccessful. If successful then you have cleared the object and you can continue.
1. Open up a new spread sheet in your excel file and run the query there.
2. Delete the spread sheet where you ran the export that caused this error message.
3. Reconnect the Excel add-in to CA Agile Central.
4. Close the Excel file, then reopen the file.
5. Open an entirely new Excel file.