Thank you for providing a document and a stack trace that can be used to reproduce the issue.
I was not able to reproduce the exact exception from your stack trace, but I have observed another one which was caused by a CellStyle called "+-". The items in the StyleGallery are screenshots of the cells of the RadSpreadsheet with all the styles applied to the cells and each cell has its value set to the name of each cell style. The exception is caused because the value "+-" is not a valid cell value. As Excel manages to open the document this can be categorized as a bug.
I have logged it in our feedback portal by making this thread public. You can track its progress, subscribe for status changes and add your comments on the following link - feedback item.
As a workaround of the issue, you could rename the cell style.
Could you try to edit the document in Excel and rename the CellStyle called "+-" to something not starting with "=", "-" or "+" and try to import it in your application and let me know if you still observe the exception you have reported?
I am looking forward to your reply.
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