I haven’t used the forge a lot so far, but I did quite a few times during the last days.
I noticed that sometimes items will just not be accepted for perfectly fine recipes, depending on what you put into the forge first. Try changing the order of the items you put in, like putting in the ring first. It may help.
I asked around and apparently it’s because the rings I had were from very early on when FoTM was just released, so they are bugged in some way. I did try to put the ring first, still didn’t work.
I got a new ring just then, infused and attuned it in the forge and the forge worked fine. Still I hope they fix this because getting all the rings I want again would be a pain in the kitten , and this was never the fault of players.
I was trying to attune my ring of red death according to the recipe. I have gathered the other required components but for some reason the infused ring would not show up in the mystic forge. Even if I drag it from the inventory the square just turns red.
