Problem:
Rhino 6 won't start with this error.

Solution 1: Run a “Repair” on the 64-bit Rhino in Control Panel > Programs and Features to correct the error.

Solution 2: If you have ShapeDiver v1.9.1 for Grasshopper installed, run the TestPackageManager command, and update ShapeDiver to v1.92-beta.1 or above.
(ShapeDiver does not cause this error for Rhino 7.)