Guys, bad news: someone else (a new LR3 maker) has now hit the error of “Unknown command: M420”! Link below. I linked them over to this thread so they could read about the ongoing efforts to solve the issue.
I told them,
Ugh. This is now the second time today I’ve read about someone hitting an error of “Unknown command: M420”! Please check out the discussion beginning at the link below, and continuing onward. (Hint: they are still working on it!)