The game didn't crash, but I was soft-locked. I tried to play Hurl on Cage of Serpents. The CoS was already in my hand (don't if that matters). I was able to save and quit. The error occurred again when I loaded again.

If it matters, I was fighting a Spherical Guardian. I am running 11 mods, but none of them should've kicked in during this fight. The only possibility would be Biomes (though I wasn't in one of those zones) or Optimize the Spire.

This is all I see in the log from the game start:
Starting game...
Controllers: added manager for application, 1 managers active
Clearing leaderboard font textures...
21:51:05.167 ERROR relics.MembershipCardInfo> Could not find discount for purchased card.
22:04:33.189 INFO OptimizeTheSpire.OptimizeTheSpire> Unneeded element iterations skipped: 467505
22:04:33.190 INFO OptimizeTheSpire.OptimizeTheSpire> Logs prevented: 3556
Finding texture: bogwardenResources/images/cards/OthersiderForm_p.png
Finding texture: bogwardenResources/images/cards/OthersiderForm_p.png
Clearing SCP font textures...
looking for file: saves\THE_BOGWARDEN_OCEAN.autosave
looking for file: saves\THE_BOGWARDEN_OCEAN.autosave
Clearing leaderboard font textures...