I find that sometimes there is an artifact after the last column of data in a CSV I'm importing that can cause a problem, so I just delete several columns after the last column of data and save the file again. Usually that does the trick.
Another common error (at least that is what I like to tell myself) is if you have two columns for the same piece of information and they are both mapped to the same Mkto field. This will prevent the import as well.