It also could be that its so low the thermostat has not learned when to turn on but the 126 error is a known fix with the bulletin.
I saw that you average temp had dropped from 38 down to 19 in just a few days. The thermostat uses an algorythm based on the past few days of temperature. So if it took 50 minutes to heat up monday and 60 minutes to heat up tuesday it will start heating 55 minutes early on wednesday. I don't know what you have your night setback set to but I would recomend no more than 3 degree difference. So if you want it at 70 in the morning only set it back to 67 at night especially when below freezing, otherwise you will pay through the nose for electric to help heat the house up.