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Since neither the plumber or the chimney guy found anything wrong I'd replace the switch. On very cold nights you'll have little to no draft initially (before the burners come on with a cold boiler) but that shouldn't trip the switch unless the switch calibration is off.
It's a pretty simple replacement that shouldn't take more than 5 or 10 minutes and it's something you can do yourself if you're inclined to. Then you won't have to battle the boiler guy over doing it at cost. There's no way you can verify proper switch operation other than the one you have now is apparently tripping when it shouldn't.
While anything is possible it's very unlikely that a damper this new would work sometimes and not others. I didn't say that the lack of a decent draft (never mentioned a downdraft) would or should cause the switch to trip. I was only pointing out the only relevant difference in conditions since you mentioned the cold weather. You'll have to go to a plumbing supply house that sell Buderus to get the right switch
Do you have any more questions?
I'd appreciate a favorable rating