It's hard to know for sure, but my guess is that your problems were caused by a combination of factors. The original tip browning could have been due to a fertilizer buildup, but more likely was a response to low relative humidity in the growing area. Most people respond to something like that by applying more water, which doesn't increase the humidity, but can rot the roots. My guess is that's what happened. I'd try putting it outside in a shady place for now, while the weather, even in zone 6, is on the hot and humid side, and letting the soil become dryer between waterings. You may also want to repot it in a free-draining medium (cactus soil is a good choice, or a soilless mix with lots of perlite), but don't increase the pot size much, if any at all.

Thank you. I am so relieved to know that I haven't introduced some deadly disease to my other plants. I will do as you recommend.