Chimney condensation

[Terroreyes]

Suddenly this winter, my chimney is sweating moisture through the brick in the basement. The condensation is right in the area where the furnace vent is cemented into it. Enough to run all the way to the floor and keep it wet all winter so far. Two changes I made, and I'm sure one is the culprit. But, which one, and what's the cure? This summer, I put a new coat of paint on the stack on the roof, and I put a chimney cap on it. I put the cap on because we had torential rains one day and it was enough to fill up the ash collection recess and run into the basement. 8 previous seasons and no problems until now. All the brickwork is fine, top to bottom.