When you want to import data into Workbooks the data must be in CSV format. Sometimes (especially if you've exported data from Workbooks into CSV format and then edited it using Excel), it's possible to save your file in the wrong format and then you'll see an error message when you try to upload the data.
For example, your Excel might default to saving the file as a Unicode file, but it appears to be a CSV file (with a .CSV extension). In this scenario, you'll be able to select the file to upload at Stage 1 of the import wizard but when you click Next at Stage 2, you'll get an error message that reads:
"An error occurred while uploading the specified file."
In the File Details area of the screen you'll see the reason why the upload failed, which is:
"symbol string may not contain '\0'"
as shown in the screenshot below (click to enlarge).
This is because the file is really in Unicode format, not CSV.
To avoid these kind of errors, take care to ensure you really are saving your file as a CSV file within Excel by choosing CSV (Comma delimited) as the file type.