Sounds like overwatering, lack of fast drainage due to gritless and water holding peaty soil. Remove it from the pot, cut back any slimy dark roots to lighter, drier parts, though don't otherwise use the opportunity to root prune e.g. for a smaller pot, at this time, but do add lots of grit, or even perlite to the mix. Try a mild house plant fungicide at half strength when you water in the new mix and never let the pot sit in drain water, though if it's propped on stones in the tray, above the water, will humidify the tree. It's best to water less often after the first flush of spring and summer, even less in winter, even if you have an 'indoor' tree, but depending on where you live, it may do much better outdoors for life.