J.A. Michel wrote:
Ya, but why would just that spot 'frost' and not the rest of the
roof? I doubt the rest of the roof is 'bad' and this one spot with
frost is 'good'. Obviously, there is a problem with moisture at that
point on the roof. Excess moisture in that spot is causing the frost.
In the last round of pics that were posted, it looked like the
sheathing was sagging and damaged as well. Could be caused by
problems with venting. You really need to have a pro look at this. One who
can investigate the cause, and correct it. There could be a
boatload of mold up there as well.
The problem will not fix itself or go away, but it will get worse!

Are you sure that the entire roof isn't frosting during the night and as soon as
the sun comes up it all melts except for that spot? Perhaps that spot has
better insulation. Does the house have cathedral ceilings (no attic)? That
would typically not be the case in bathrooms so they would still have a space
over them.