Check if your drive has been formatted with AFPS. (Supposedly this automatically occurred with the upgrade to High Sierra, probably still the case with Mojave) If so you either need your project moved to a drive which is not AFPS, or you need to move to a Unity version with AFPS support. So that would be 5.6.4p3 or later.
This is my guess, as the original issue isn’t about the editor not loading but instead assets disappearing.