Cattleya roots dying suddenly

[mam288]

Hi, I have a favorite cattleya that I've been having some trouble with. Several years ago I got a plant form a grower that was health and bloomed a couple of times for me. It had a really nice root system and was doing well, or at least I thought it was. It then started to look unhappy and when I checked, all of the nice roots that had been healthy not long ago rotted. The plant followed shortly after. I thought it was maybe that I hadn't repotted it since getting it from the grower but never knew what actually happened.

I got a replacement plant last summer and had the same thing happen again! I repotted it back in May and it had since put out a couple of new growths and the roots seemed nice and healthy. I noticed the new growths were not able to support their weight anymore and were drooping and when I took it out to check on it, all of the roots had rotted. I couldn't find a single live root. I have since put it in a bag with some sphagnum moss but am wondering if anyone has thoughts on what might have happened. I have not changed my treatment of the plant, it is in a window under grow T5 grow lights. It's potted in an orchid bark mix, which seemed to still be in tact when I took it out of the pot. I water it about once a week so it should be drying out between waterings.

I'm attaching some pictures. If anyone can help it would be much appreciated! I'm stumped and don't want to lose my favorite plant again! Thanks in advance!