AdoReno

Final update: I'm an idiot.
 
So, I had this problem again while installing a dlcpack, and I decided to test things out. I removed that specific dlcpack and loaded the game again. It worked. I wasn't sure what the issue was with that specific dlcpack, so I began looking it up for a lot longer than I did before. All the answers mentioned the addon gameconfig. "Weird," I thought, "I already have that, and the only difference between each version is the number of traffic cars and peds that spawn."
 
Then I realized something.
 
You know how I said I reinstalled my mods folder?
 
I forgot to reinstall gameconfig, because I mistakenly thought I already had it installed. I forgot that, unlike heap adjuster and packfile limit adjuster, gameconfig has to be installed to the mods folder.
 
So yeah, for anyone else who sees this, that's probably the solution. But still, don't fuck with your mods folder.

Okay, I fixed it. It was a duplicate in UltimateBackup's CustomRegion.xml that I swear wasn't there when I ran through it twice to ensure it was clean. I'll keep this here in case I find a third game-breaking bug that turns out to be the smallest shit.
 
This game, man. I swear.