White spots on wood fiberboards above the rafters

Hello everyone,
we have been living in our low-energy house for 12 years. For 3 days, there has been a musty smell in a room facing north. I just climbed onto the roof and looked under the tiles to see how the wood fiber boards look. There are white spots everywhere, and I don’t know what they are. Hopefully, it’s not mold.
The roof is constructed as follows:
Plasterboard on batten framework
Vapor barrier
Rafters
Insulation between the rafters with space for ventilation
On top, wood fiber boards
and finally a "foil"
Battening
Concrete tiles
Can anyone tell me what these spots are?

Thank you very much and best regards
Wolfgang

That is definitely mold. Do not ignore it and look for the cause; something is leaking somewhere.

This is definitely mold. Do not ignore it and look for faults somewhere, something is leaking.

My layman’s guess is also mold. Since you apparently are not a professional in this matter, I would get help in searching for a leaking spot and have it determined whether the cause lies in shoddy construction.

After 12 years and still white mold, I rather suspect that something was changed, perhaps retroactively a photovoltaic system or a defective roof hatch, etc.

Nothing has been changed. I am surprised that the infestation is spread over the entire, rather lower half of the roof. If there were a leak in the vapor barrier, wouldn’t the infestation be more localized? The vapor barrier was properly sealed and has no holes.

Thanks by the way for your answers