Re: This is a serious problem.
I suspect he let frost build up around the control which physically prevented the mechanism from moving. It would be near the door of course, and thus subject to frequent bursts of warmer, moist air. Melt, refreeze, repeat for years on end.
Could have been made worse by not shutting the door fully each time he was in...keeping it only slightly ajar wouldn't hurt the temperature inside too much, but would allow moist air from outside to settle around the control