Ragemonger is part of that bearded guy fusion that runs now. That may be part of the problem. He is marked as part of the fusion and there is some sort of bug right now, as far as i know, that prevents you to use champion that is part of the fusion and have any equipment on him.

Strip him of all the equipment, if he has any and try again. Also, unlock him from tavern, not from when that confirmation check about him pops up.