Chas, if the bad leaf is on an old cane it could just be the natural senescense of the cane. They do eventually drop all their leaves and the cane shrivels up. My Phal type Den is doing fine in less than optimal RH. My house only runs around 60% on a good day in the summer, a little higher in the winter. It's blooming for a second time since December.
HA! 60 % RH!!!! I would die for that! It's about 30% or so today. Here in Colorado it's just the opposite. We have a higher % in the summer and less in the winter. We get the monsoonal flow coming up from the Gulf of Mexico in the summer.
LOL, the 60% is ONLY in the tiny micro-climate I have created in my bedroom bay window. There are 75+ plants in that small area plus drip trays. The rest of the house is around 30% and hotter than the hubs of hell in summer.