Error on the Import Mapping page

How to handle this error on an import Mapping page:

Error, ExcelImporter, constructor: Unspecified error (Try changing the Excel worksheet name to 'Sheet1').

It seems like Excel sometimes stores its data internally in a different format than usual. The import functions cannot open and read the file in that format. At this time, we do not know why Excel occasionally does this.

The simple solution is to copy the file contents to a new Excel file:

1 - Create a brand new Excel file with a simple filename (no spaces or unusual characters).
2 - Keep the default worksheet name (Sheet1).
3 - Copy the header and data rows from your original Excel file to the new Excel file. To copy and paste everything easily, right-click on the small blank rectangle above Row 1 and to the left of Column A.
4 - Save and import the new Excel file.