I was unable to upgrade the Nightingale Armor Set even though I had Void Salts in my inventory. After about 30 minutes of trying to figure it out, I finally did. I had a Quest Item, Fine-Cut Void Salts, that were causing me to be unable to upgrade the set using normal void salts. After turning in the Fine-Cut Void Salts to the proper person, I was able to upgrade the set. Hopefully this will help someone with the same problem out in the future.

I have experienced the same issue. I had 3 regular void salts as well as the "Fine Cut Void Salts" in my inventory.

Thanks for posting this fix. I will try getting rid of the fine cut void salts.