So, I tried to upgrade some equipment and it will show a Battle Power increase, then generate a browser error and ask for the page to be reloaded. When you return, 1 grindstone is missing. I have only tried using 2 grindstones. It has happened to me 3 times. No gold appears to be missing and the item definitely is not being upgraded. All items attempted (3 different, I believe) were green
A minor update... This error could possibly be because the grindstone isn't valid. I believe I get the grindstone as spending award for spending over 100 diamonds. I believe I have collected this same reward several times even though I haven't spent any more diamonds. It may be a glitch and since the grindstone is invalid, I am getting an error. Just a thought.