My experience is that kefir that is not fermentet long enough give more rashes, itching and so on. In the same areas as you describe.
Cow milk yoghurt does too, and most other stuff that contains some form of processed/pure sugars (milk: lactose, kefir: cane sugar).
I have not tried buckwheat yet, is this antifungal too? Seems like I missed something 🙂
Overall, every carbohydrate in a refined form (white rice, plain sugar, white bread) gives rashes, even when it’s in one of the “good” products like kefir, yoghurt or porbiotics.