Gengis,

Is this under artificial lights? Even if you haven't changed the exposure, if it's only getting natural sunlight, it's getting much less now than it did in July -- some leaf fall is a natural response to this. If it's indoors, the humidity is probably dropping too, so this may be the cause of leaf browning; using a pebble tray, humidifier, or other way to increase humidity will definitely help. Remember, this is a tropical tree, and northern houses are not even close to the same conditions!

Finally, if it was outdoors during the summer, and indoors now, expect some leaf fall as it reacts to the change in environment. If it's still outside, bring it in! The literature may say it can survive short frosts, but you likely have a young tree in a pot, right? It needs protection!

Good luck with it!

Jim

PS, "No obvious insects" doesn't rule out hidden ones -- check the undersides of leaves and new shoots for tiny webs (spider mites) and ease back on the watering schedule -- it won't be growing as much now, and too much water can lead to root rot.

Thanks Jim,

don't worry, my tree was brought indoors. I'll try to increase the humidity and lighting. I really don't see any insects, but I can't rule-out a viral infection.

Thanks for you help