Alright, I figured it out. This will be patched soon. It's caused by a lootable that has been previously set as a persistent container, and the data was saved to disk so it can be restored. The problem is that someone/somehow that lootable has been since deleted / changed, so the definition of said setting cannot be found.
Please follow these steps to fix it for now:
Please beware that this will wipe all persistent containers.
Unload the plugin
Delete the oxide/data/StaticLootables_data.json
Load back the plugin