We have the same issue since installing a humidifier. The water condensates on the inside of the pipe (validated with small camera). It is so bad it leaks out of the metal pipe at the bend going into the fan hence it stains the ceiling as well. The water isn't insignificant sometimes during cold snaps. The weird part is it only happens in two of 3 bathrooms. In any case, tried everything. Insulated the pipe, checked the damper, everything is normal. I'm now back to the idea of some kind of vent pipe material that is better suited or to build a condensation trap of some sorts.
I was having the exact same problem as you. Fan is insulated, pipe is insulated, still dripping. I did 3 things that seemed to have fixed it:
1. Instead of just laying the existing insulation over the top of the fan, I took up some new dense insulation and covered all sides of the fan housing with it so it's directly against the metal. Generally, just did a better job at insulating it.
2. I vented the fan straight up through the roof with a roof vent and make sure the pipe was insulated all the way up.
3. I have a Panasonic Whisper Green that has 50/80/110 CFM. I changed that setting from 110 to 80 so it wasn't pulling quite as much moisture. It take a little longer to clear the steam but... NO DRIPS!
Update: Well, it still drips when it's especially cold, but at least not as much. Yes, very frustrating.