Rob, it may very well be the softened water. If the water softener is the type that uses salt to soften the water, that would definitely be your reason. The salty water will kill your plants.
I would tap the plant out of its pot and have a good look at the roots. If they're still white and green and healthy looking, just switch to using rainwater - or even water from an outside tap that isn't softened (though rainwater is the best, really, as you know). If the roots have started to die back, you'll have to cut off the brown, mushy ones and replace the potting medium so that no residues are left behind. Give the remaining roots a good washing off as well. Then repot. It may take some time for the plant to fully recover, but if it has any new growths (mine is putting them out now) then new roots will grow and you'll be back to normal in a year or two.
I hope this helps! Any questions or if anything comes up when you look at the roots, let us know! Cheers.
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