Enemies and player are invulnerable. I already had this in different monos and bossfights. You have to restart the game to fix it. In the video below the game also crashed after a certain time.
I am hitting this a few times a day. Seems to be when I get crit and all my ward is depleted. It then doesn’t allow any more ward to build and as you say both myself and the mobs are invulnerable at that point. Have to quit game, or wait a bit and it’ll usually just crash. Should mention I am not using warpath.