This behaviour has been tested on the latest GA version of PPM, as of this writing - 15.4.
The reason this error gets thrown is because, Ranking Rules can be run only on values that are quantifiable. The application tries to convert the values present in an MVL to a quantifiable value such as a Boolean, or a Number, and fails during that attempt at conversion. The app-ca.log indicates this. MVL values are not quantifiable, because of the nature of data that they hold.
Use only quantifiable data to run Ranking Rules on. To this end, mainly numeric attributes should be used in the Waterlines to run Ranking Rules on.