Fresh milk curdles when added to hot coffee and tea
Boil the milk on its own in a clean/rinsed container (microwave).  If it curdles, it's the milk. 

Otherwise either there is a decalcifying agent in the kettle or something in the coffeee is making it too acidic.  It takes very little, after decalcification, we usually have to pass about a gallon of water through the coffee machine before the steam wand stops curdling milk. 

If you're in a jam, boil your water in a cup in the microwave and avoid the kettle.