Progression Bug - Weapon Pieces Act 4

After clearing the necessary bosses: "The Prisoner", "Scourge of the Skies" and "Diamora" multiple times Arastas is still closed for me because I dont have all the pieces of the weapon which should not be the case.
I also noticed a Quest-Icon on the minimap when I am standing in the Bossarea of "Scourge of the Skies" but there is nothing.
Hopefully this Problem can be solved because it blocks me form progressing any further in the Game.

Thanks!