I've tried the following things and it consistently crashes sometime during the 18th of Obsidian (the save is on the 17th):

cancel steel making, thread spinning, melt orders
cancel wall building at lava trap (down deep), stopped phyllite hauling/minecart stuff where lava trap is
killed guy with strange mood (I used exterminate from DFHack)
cancelled all jobs, forbid EVERYTHING (all z-levels) except food/water
cancelled all jobs, forbid EVERYTHING (all z-levels) except food/water AND I burrowed/stationed everybody at the food/water
exterminated every creature on the map I could find, all races/types (DFHack again)

this is DF v50.09 and I have DFHack installed in Steam as well.

I'm also running a handful of mods from the workshop:
Squad Burrow Icons
Seasonish Crops
Reduced Z-Level Fog
Obvious Engravings
More Food Names
Interface Tweaks
Deon's missing Plant Graphics replacement
Work Detail Icons
See-Through Smoothing Designations
Audible Alerts