The oldest growths on Paphs die back like that, so I don't think that there is a particular problem on that count
I know that it's normal for older leaves and older growths to turn brown. But that is exactly the problem! It's the latest growth that is browning. I do think i'ts rot because it's soft, not dried out.
See the growth that's starting to rot is the one with the current peduncle and flower.