I can't help but they look like possible rotting to me? I believe damage from mealy bugs may 'help' to fungal disease too.

Pull the plant out of the pot and clear the soil away from the roots to see if you have root mealies. That is exactly what happened to mine one year.

I'd guess what Kara said is correct, rather than the second most-common problem (root mealy). More air circulation and less water now (if you're in the Northern Hemisphere), but you might also need to make the soil more porous.

In my view it's lack of nutrients, as what I sees always the oldest part is affected .

Try a foliage fertilising .

The same thing happened to my favorite Stapelia Scitula. Take one of those yellow "fingers" and pull it from the base, so you get some of the roots. I'll be you will find mealies all over the roots. They are hard to fight, and the only effective treatment seems to be a systemic