Hum. I've had the same problem this summer/fall/winter. One plant was given to me completely dormant. It had a beautiful caudex, and interesting limbs that were hard as wood. I tried and tried to bring it out of dormancy, but it started rotting from the ends of the limbs. It never did grow leaves, one day the whole plant was goosh, and not the brown stinky stuff from over watering. It was rather yellow, and dry-squishy. Boo hoo,
Other plants are trying or have done the same thing. Weird!
Your plant, I would dip your cutting tool on isopropyl alcohol, to sterilize it, and cut an inch below the rot. If the stem's insides have any brown, cut a little more, sterilizing your cutting tool with each cut.
I take it the first photo was it blooming happily. Maybe it didn't want to go dormant, it looked so green. To abruptly go inside from a more humid outdoors might have put it into some kind of shock.
A story here--- I visited my mother, long ago, in Virginia one summer. It was boiling hot outside, 101 degrees with 95% humidity. We all stayed inside mostly. I am accustomed to about 60% humidity. After three days of that dry air conditioning, my sinuses hurt, then they started bleeding! I had a bloody nose for the last four days of my week's visit!
So, do the cut, and give the plant a small drink, maybe 1/2 a cup, every day for three days. Ha, how's that for advice? The water will move around in the potting mix some. Better sips than a flood. Let us know what happens, OK?
