Yes, I think you are right. In the summer, I replaced the old attic fan
with one of those "all-season" ones that have the humidity sensor. The
humidity is quite high up there as I activated the sensor at 65% and she
is running. The problem is that the outside humidity is 70% so I'm not
sure if it will ever turn off.

There is no frost present. How do you prevent condensation when the
outside air is humid, the attic air is humid, but the surfaces are very
cold.

Block the vents, and run a dehumidifier. When it gets cold enough
so that icing on the roof looks like a realistic hazard, shut it off
and unblock the vents.