Frost/ice on the inside of the coop?

If there is a big enough difference between indoor and outdoor temps, you can get frost even on insulated walls without there necessarily being problematically-high humidity (although if that goes on for long, it can *create* humidity problems by preventing your vents from doin' their stuff, since so much water remains captured on surfaces rather than in vapor form in the air). I am doubting that's your issue though.

Even with just a 10 F (or so) difference in vs out you can get frost on THIN uninsulated things, like windowpanes or exposed sheet metal, without there necessarily being problematically-high humidity. This is not generally a problem as long as it is only a small area, e.g. if it is just a window or two it is ok but if it is much of the inside of the coop then see above about moisture trap effect.

OTOH if you are getting frost on the inside of insulated surfaces, despite a non-huge difference in temperature between the coop and outdoors, it generally DOES mean you are dangerously humid in there. And from what you say, without being there to see myself, I'd be inclined to suspect you are heading somewhat in this direction.

I'd suggest reopening the vent if at all possible, and doing a good coop cleanout to get rid of any damp (even if frozen) areas of bedding and deposits of poo, and replace with fresh dry bedding. (Needn't be a total cleanout unless you want to or think it needs it). Make sure the waterer is not spilling or leaking. Then give it a day or two at those conditions and see how it goes.

In the meantime it probably couldn't hurt to smear a thin layer of vaseline on the chickens' combs, perhaps at night by headlamp when they are on the roost; many of us feel this does do some good vs frostbite although I am not aware of any actual studies testing it.

Good luck, have fun,

Pat