I just found a bug, where after you die and reload in campaign you cannot meele. You have to completely restart the game in order for it to work again.

yep. same here. did a meele (out of invisible mode) and died.. after respanw it doesn't work. neither the keyboard nor the mousebotton,
need to restart game. leaving and rejoining the campaing doesnt fix it.

had that a few times. just try to switch weapon and try to melee again.