Hello! Well it looks like the actual crown of the phal--the top of the plant where new leaves emerge--is fine, which is good news. What you have is some kind of leaf rot, probably due to a fungus (or bacteria). You can try spraying with a fungicide. In the future, just make sure not to let water collect in between the leaves, and don't let the wet sphag come in contact with leaves and the base of the plant. Maximize air movement around the plant, and don't water in the evenings or at night. Also be careful not to overwater (I tend to do that with phals potted in sphag so that's why I prefer bark).
Another cause of leaf drop is a compromised root system, so since the pot is transparent, you can take a good look at the roots. From the photo, the roots look great, so I guess this is not the problem. Good luck!