I took a bunch of long rest, cause I already kinda gave up on this save and just kept on freely exploring. I've walked around hoping I'd find Minthara preparing an attack somewhere, but that was just wishful thinking.
When I found Halsin he told me to kill all the leaders, changed into a mouse and left (if I remember correctly). The logbook told me that I freed him, without a word about his death. What's even weirder is that when I talk to Zevlor I can only tell him that Halsin is still trapped by goblins (that are all dead now - Also though that it would help)
To be honest I'm completely out of ideas on how to unbug this.