De-lurking--

I was wondering if anybody has heard of this and what caused it? We have a recently (august) remodeled kitchen with an OTR microwave. It is exhausted outside (it is on an exterior wall). Here in Chicago we have had a cold snap, zero degrees, and we recently noticed the microwave has frost inside of it (like a freezer needing defrosting). In addition, the surrounding cabinets have moisture and frost on the surfaces adjoining the microwave. Please help, as I am afraid of the damage to the cabinets. Is it some sort of problem with the damper? Thank you very much!! P.S. Maybe we should just close the hole and switch to recirc fan?