MBM v8.5.5
OS X 10.11.5
Similar problem, importing via clipboard from either Excel or via a text editor - except MBM actually crashes. I have a fairly small list (under 1000) but repeated attempts to copy and paste in one go always crashed it (after periodic spinning wheel for a few seconds at a time). I finally got them all in, by importing 300 at a time and waiting before doing the next batch. I'm guessing this is probably another manifestation of the same thing?
Screenshot of error message attached.