Smallish plant, one or two years from flowering, I guess.

It is dropping leaves. It had nine or so leaves, there are now five
green leaves and two that are turning yellow.

It is now winter. The indoor humidity is about 40-50% at a steady
22-23C. The plant is getting about 35kLux of sodium-based (yellowish)
light. I water it twice a week, the roots look ok to me.

Help! What is happening? I haven't changed anything AFAIK. The
plant has been happy the last fourteen months or so.

Geir