@1337n3ss Most growers assume that because sphagnum peat and coir look similar, that coir and peat should be interchangeable, but such is not the case. Coir and peat are different both physically and chemically to the degree where growers should have work-arounds in place to deal with the differences. Coir is mildly to strongly allelopathic to a high of plants. Allelopathic plants release biocompounds toxic to other plants as a method of reducing competition for resources. Coir is very low in calcium, making it essential to ensure the plant gets an adequate supply of calcium from an extraneous source and an appropriate amount of magnesium (magnesium is a calcium antagonist). Coir, though very low in Ca, has a pH low enough to preclude use of calciferous liming agents as a source of Ca/Mg. It is extremely high in potassium such that fertilizers with normal amounts of potassium (roughly 60-65% of the amount of nitrogen in a fertilizer product) can create a potassium toxicity. It has much less loft than sphagnum peat, so it compacts easily whenever watered from the top, which increases water retention at the expense of aeration, less than ideal for plants like sans that do best when treated like a succulent.
in all other cases all rotting leaves were young (unfortunately, I did not take any pictures before cutting them off). What could be the reason behind that? If you're not fertilizing and/or the fertilizer you're using doesn't contain calcium, coir's low Ca content is likely to blame. Ca must be in the nutrient stream in adequate amounts as new cells form. A deficiency of calcium causes cell walls to form imperfectly and/or weakly, subsequently causing rupture of cells or areas where cell contents spill out into inter-cellular spaces, leaving rotten areas which act as openings through which fungal or bacterial pathogens can attack the plant.
Low fertility, especially deficiencies of the mobile nutrients (nitrogen, phosphorous, potassium, magnesium) can force the plant to rob nutrients from older leaves during a period of resorption, the first step in the shedding process, but I suspect (given what info you have provided and the images) that this isn't what's causing the conspicuous symptoms.
Do you fertilize? with what? when last?
How do you tell when it's time to water?
The pot has a drain hole?
When you water, do you water copiously, so at least 20% of the water used to wet the entire soil column exits the drain hole?
Does your home make use of a water softening system - the type that uses salt as the exchange medium (ionic exchange water softener)?
Al
