You're using other characters other than letters, numbers, commas, periods, parentheses, and hyphens.
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A cell exceeds more than 50 Characters.
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Rows that do not contain at least the first name.
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Matters without a contact associated to it in the same row of the CSV file.
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Other matter items, such as notes, that do not contain a Matter: Name in the import for that row.
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You have custom fields set to *Required and the import does not contain some, if not all, required custom fields in the excel sheet.
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You're using other types of custom fields other than text field.
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Not capitalizing the first letter of the Matter: Status column.
It should be: Open, Pending, or Closed.
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Date formats need to be: dd/mm/yyyy