When the milk curdles it is likely to be because your mixture is too acidic. Milk + acid + heat will always curdle (ricotta cheese). The source of the acid could be that your milk is approaching its use by date, or your caramel is darker than usual, or your sugar has some residual molasses (organic, etc.). I’ve also found that pasteurized (rather than ultra pasteurized) milk is much more likely to curdle.
Regardless of the source, here’s a trick that works for me: add some baking soda, about 1/8 tsp to 1/4 tsp, to the milk while it is heating (before it’s poured into the caramel). Works like a charm.
So you can either remake the caramel with baking soda, or you can blitz it in the blender and smooth it out that way. I don’t know if the blender method will give you a perfectly smooth buttercream or not, but it might be good enough and help you to move forward and not have to remake it.