Cymbidium ‘claret x vieux rose’ dropping green leaves!? Help!

[MDBuch]

Hoping I can get some insight on best courses of action!

This was given to me by an experienced grower about six months ago, and now I’m having this problem.
Below pics show how the green leaves appear to have rotted at the base? The “sheath” the leaves are growing out of appears to have turned brown first.

I have another cym. that had new growth do basically the same thing after growing about a foot tall. Ideas? The two plants were growing about eight feet away from each other with still healthy unaffected plants growing between them.