I've had this quest break at least a dozen times and through some trial and error the problem has always been the same : if at *any* time between getting the 'poison' quest from "Book 'em" Danno and the party arriving at the Fentan house, someone in the party is approached by a non-party NPC who has dialogue regarding another quest.
This has included :
- the messenger from the the Keep (PC stronghold quest)
- messenger for Annoyman to go to Radiant Heart
- Jans' cousin
- Dermin
Nowadays I try to make sure Mazzys' quest doesn't overlap with any others - and keep a few extra save games just in case
