Just to add to this discussion, I’ve been running a licensed copy of Rhino 6 For Windows (the most recent version) within Parallels Desktop 14.1.1(45476) For Mac Pro Edition on my Late 2012 - 3.4 GHz - Intel Core i7 - iMac (macOS Mojave version 10.14.3) without any problems whatsoever for the last few months until recently.
So apparently a recent change or a series of changes to the software applications involved has finally put the kibosh on Rhino 6 For Windows (P.D. virtual machine version). 