CSV LookUp Plug-in fails when your CSV file contains blank or empty values in the last column of the row.
The CSV file contains a blank value in the last column of at least one row.
The CSV file cannot contain blank values at the end of any column row.
Should any row contain a blank value in the last column of the CSV LookUp Plug-in it will fail as a value is expected.
The CSV file can contain blank values in the middle columns as long as that column is not defined as containing a key value.
So the solution is to ensure the last column of each row contains either a valid value or a space or a dummy/fake value.