Making ice cream and fats don't dissolve in base
Looking at the ingredients in step 3, my guess is that the sour cream is causing the milk to curdle, possibly made worse by the heat from the melted butter.

You might try experimenting with first mixing everything except the sour cream together, then letting it cool completely, and then slowly whisking in the sour cream.

You might also try slowly heating the lumpy mix you already have, and see if whisking it over a low heat helps it to dissolve.

Good luck.