Are you sure it hasn't been watered too often? When that happens, the bottom of the trunk gets soft and begins to rot (take a good look at it and maybe pull away some of the soil), as days are getting 'shorter' due to fall coming (I know it's indoors) and daylight decreasing, which means less water is needed by the tree. If that's what happened, you may be quite out of luck as it's not really a fixable thing - if the roots are wet (though surface soil is dry) then your soil needs a lot of grit added to it as it's not draining fast, which it should be (NIP's don't like wet conditions). In those circumstances, a fungicidal 'drench' could be applied after slimy black root parts are cut away, but if the trunk is all mushy, then you may be in trouble with it.

You can always take it out of the pot and examine the (root system) and as LUCY says maybe when it was repotted you buried it too deep. Anyway, as in a recent thread, if it isn't getting a lot of sun/artificial light - that too could be the case. Fertilization doesn't seem to be the case if the plant is that old with that height - seems normal (in height). But yah, root rot/'trunk rot' sounds pretty good right about now. 'Foul-smelling' soil will be an indicator of a problem. Many factors seem reasonable.

Dax