Thank you kindly. I'm continuing to look at this too, and I shall post whatever I can find.
EDITS:
Crash conditions:
After 1 battle, not doing anything else
Accept all available missions, then 1 battle
Accept all missions, fail 1, then 1 battle
No Crash conditions:
After 2 battles, not doing anything else
Accept all available missions, after 2 battles
Accept all missions, fail 1, then 2 battles
Conclusions: It seems to only happen after the first battle. I have experimented with removing the complete_quest script from the event but it still causes the crash.
Also, the NoMethodError occurred error came up a few times. It seems to have something to do with the "Victory!" display at the end of the battle. I change the value on that line from true to false to nil, and the error comes up regardless of which value I put.
Also also, when I talk to an NPC, the first line of the first box will always be cut off. When I talk to it again, it will be correctly formatted. I think it has to do with the ATS, but I don't know how to fix it. The names will be cut off too when I make a \nb (for now I'm just fixed it by adding a bunch of extra space after the name ex. \nb[Name ])