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We're seeing the following error when trying to preview some .xlsx files. They're more complicated than you're typical Excel workbook but FileVista supports Excel previews so expected them to work. The error references to PDF are confusing. I don't believe there are any linked or embedded PDFs but I'd have to ask the author.
Server Error in '/' Application.
Startxref not found
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.