Is your fan vented correctly? Some people who install these fans just
vent them into the attic, and it could be the discharge vent is blowing
right on the sheetrock.
You said when you looked in the attic there was insulation everywhere.
Did you pull up the insulation where the leak is occuring? A lot of
times the insulation can trap water causing it to pool on the
sheetrock.
This is what happened to me yesterday. There was a leak in my kitchen
ceiling due to the heavy rains in NY and when I went into the attic to
look, I could not find the leak, until I pulled away one of the bats of
insulation and found a big puddle on the sheetrock. Turned out there
was a small leak in my roof right where a roofing nail was portuding
in.
Another idea, is there a way to take a shower without using the fan
just to see what happens?