I am also puzzled by the neutral conductor burning, but not the hot
conductor. Assuming the neutral had a tight connection it indicates to me
that the neutral was carrying more current than the hot. This would only be
possible if the neutral conductor is shared by another circuit which is also

You didn't move the 50A breaker in the main panel, did you?
If you've somehow got both the hots feeding the subpanel
on the same phase, that would overload the neutral, too, and some
panels allow you to do that. (It would also explain
why the AC unit croaked.)