Help! Repotted and roots died

[Jfrisch]

Hello,

I recently repotted two seemingly healthy (slight root rot but nothing crazy) phals in a mix of sphagnum and bark and after 2-3 weeks most of the roots have completely died. They are now just casings of what they used to be. What happened and why so quickly? I have been watering once a week with a dilute fertilizer. I now put one phal that has little to no roots in a freezer bag with a pinch of moist sphagnum to see if I can encourage root growth. I have moved the other phal to a dark corner of my bathroom in hopes of providing it with more humidity.

I also recently moved a pap to an east facing window in a higher humidity room and it developed crisp brown spots on some of the leaves. Any idea why?

I am confused because I have had all of these plants for over a year and they have been thriving. Could my potting mix be contaminated. Can Physan 20 destroy an orchids roots?

Thanks for any feedback.