The attached file was generated by .Cells 22.214.171.124. When opened in Excel 2010 it brings up a red error bar that says "Protected View. Office has detected a problem with this file. Editing it may harm your computer. Click for more details." However, when we tell it to allow editing anyway, the file seems perfectly operational, and we can save it as an XLS and open it again without any problem.
We have a suspicion that this may have something to do with validation. We have seen this problem when we put validation ranges on one worksheet but the validated cells on another. Apparently later versions of Excel (XLSX) allow this but Excel 2003 (XLS) does not. When this happened, we would get this notification on opening but ALSO an error on SAVING. Now that we've moved the validation ranges to the same sheet, the XLS we save is perfectly functional despite the warning on open.
We're afraid that this warning will scare our customers.
The Inputs sheet contains both the validated cells (e.g., H19:I24). These are being validated against $F$324:$F$327 on the same worksheet. The validation appears to work just fine.