1. Reduce the size of long fields. This could be the header fields are too long. Change them to something simpler like Name and Email.
  2. Remove any apostrophes in the file. These can be in people's names of the imported list, and will mess up the file reader. Perform a Find and Replace in Excel to replace any apostrophes with a space character.
Save the file and try to import the saved file as a new separate list. The original list that generated the error can be deleted.