So I turned the darned cooling units off, and temperature became a stable 66 to 72 year-round in my nicely insulated cellar. This was excellent and the insulation was not even R-30 as it should. Probably around R-20. Temperature fluctuation having been solved, the remaining problem was humidity, or actually lack thereof. Winter cold means dry air.
Dry air means dry corks, which eventually come to differ little from sponges.
I had initially installed a Hermidifier humidifier. In other words, the maintenance-free humidifier had to have been cleaned regularly. All this despite that the water line into the humidifier had a filter. Designing the new wine cellar produced two pleasant surprises, one on insulation and one on humidification. I found an insulation contractor who can spray some foam polystyrene inside the walls which produces insulation of an R-30 value with only 4.5 inches of material.