Hi Libby and welcome to CS.
My roasts of this bean (from Nov Bean Bay) also resulted in a "sour/fermented" taste. I tried a few different roast styles, same result. So, sorry - no cure from me.
I didnt like it at all, although a few other people who tasted it thought it was very distinctive and nice. Ive put the rest of my Sidamo to the bottom/back of my pile of green beans and may pull it out at a later stage to try again. Or maybe not - this has been my least favourite bean ever.