Connect with vegetarian and vegan friends from all over the world.
The coffee is to acidic for the milk. The hotter it is, the less acidity needed to curdle the milk. Heating up the milk won't help. Just let your coffee cool a bit or get less acidic coffee. I've never had the problem with curdling with my non-dairy products. Also check for stabilizers in your milk, which should prevent the curdling.