I can replicate the crash with your save and I'll work on figuring out what's causing it. It has to do with map generation but it's not consistently happening on the same map script, which means it's something randomly generating wrong, specifically for the RNG seed in your save.
Two things you can do on your end to work around it: first off, I wouldn't recommend ever using ironman in modded OXC, precisely for this reason. There are bugs, some of which will prevent ironman campaigns from progressing, which can be prevented by having backup saves and the second recommendation I'll give. You can turn off ironman mid-campaign by loading your save in a text editor, finding the line that says "ironman: true" and changing it to "ironman: false". The next thing you can do once you've turned off ironman is to go to Options>Advanced>General and turn "Save Scumming" to "Yes". This will reset the RNG seed when you reload the game, which will re-roll the terrain and map generation, meaning you can start the terror mission. You can turn it off again once the mission has started if you like.
Edit: I've found which map script the mission is using, you can replace the file in your rulesets folder with the one I'm attaching and that will also fix your crash instead of doing the RNG manipulation.
