I think you are right about the reservoir. When we got up this morning, the water again didn't work, and the fridge temp was 10 degrees. We left the door open for a little while and then the water started working again. Asume something thawed. But a leak by the filter now appeared (upper right of fridge) - drops and also a pin hole one spraying. I removed the bypass, and then I had water everywhere. So I turned off the water. The place where the filter attaches is rather poorly engineered and I wonder if by removing the filter, I've got two problems now - something causing the freezing up and also a water leak at the filter.
You mentioned the plastic flap - did you want me just to check to see if it's there? I do see a area behind the drawers, but no plasic flap. Not anything we've removed in the last few days either.
How hard is it to get to that part? Do you access it from the fridge side? If I do get it out, is there a way to test if that is for sure the problem? Hate to spend 40$ on a part and just hope it was the problem.