Pelin

Once they are healthy again, if you still want some mineral content, add it back one variable at a time: a small piece of dried fruit or a very small proportion of unrefined sugar, and keep the rest the same so you know what caused what.

That is the habit worth taking from this. You changed one thing three weeks ago and could name it immediately, which is why the diagnosis took one comment. Change two things and you would still be guessing.

The sugar was the whole problem and the smell has a name: butyric acid, which is what rancid butter and vomit have in common. Certain bacteria produce it when conditions favour them over the normal yeast and bacteria in the grains, and coconut sugar is one of the commonest triggers people report, along with very dark sugars and molasses.

So something tipped the balance of your culture, and you know exactly what. Plain white sugar is the boring reliable answer.