The Trail of Corruption - Cannot craft the Horn in Act 5 (Act 2)

The title says it all.
I cannot get any of the needed quest-items. Starting with the husk.

I've tried killing the two bosses in the Bone Pit solo more than 10 times already. Also tried it multiple times in a group (with 2 and 3 people) about 5 times as duo and 6 times as trio.

I've/We've tried it in following ways:
Killing mage first, mastrodon 2nd
Killing mastrodon 1st, mage 2nd
Tried to kill both at the same time

We tried the kill 1st and than 2nd ones atleast twice (if not more), trying to kill both at the same time atleast once.

The musk (quest item) did not drop.

Same with the titan. No quest-drop here.
Also the game telling me that I now need to craft the horn at the caravan, but I do not have any quest-items whatsoever and no NPC is telling me that there is a quest to be completed here.

Where to proceed now? I'm hardstuck here right now.