Did you taste the milk before heating it?
I actually went down stairs, heated a cup of milk with 2 tbls. of chopped fresh ginger, letting it come to a boil and it was fine. So it makes me wonder if your milk was starting to turn sour, heating will force the curdle.
Many years ago, a cup of hot milk with ginger steeped in it was given to young ladies when they had monthly cramps...... just a little trivia for you.