My first guess is that the attic is UNHEATED and the hot, moist air from the bathroom is condensing on the cold duct in the attic. Hence, it runs back into the bathroom.
You need to insulate the duct. As long as the duct isn't cold, then you should be OK. The best thing to do is go to a Home Center and ask for what you need. DO measure the length and diameter of the pipe. DO tell them you have a 45 degree turn.