Category cannot be found

I'm using the UCC/UIS integration with a new project. I set the character up with an Assault Rifle, a Bow, and a Grenade by following UCC/UIS videos, and compared all components to the integration Demo. The character has no items on Loadout, and there are 3 items pickups.

Everything worked fine the first few times that I ran the game in the editor, and the only things that I changed were some Animator transitions, and assigned some Surface Identifiers. But then I began to get the following error when entering Game mode:

Error: Category with ID 3636424895 cannot be found.
UnityEngine.Debug:LogError (object)
Opsive.UltimateCharacterController.Inventory.ItemSetManagerBase:CategoryIDToIndex (uint) (at Assets/ExternalPackages/Opsive/UltimateCharacterController/Scripts/Inventory/ItemSetManagerBase.cs:90)
Opsive.UltimateCharacterController.Character.Abilities.Items.ItemSetAbilityBase:Awake () (at Assets/ExternalPackages/Opsive/UltimateCharacterController/Scripts/Character/Abilities/Items/ItemSetAbilityBase.cs:46)
Opsive.UltimateCharacterController.Character.Abilities.Items.EquipUnequip:Awake () (at Assets/ExternalPackages/Opsive/UltimateCharacterController/Scripts/Character/Abilities/Items/EquipUnequip.cs:97)
Opsive.UltimateCharacterController.Character.UltimateCharacterLocomotion:Awake () (at Assets/ExternalPackages/Opsive/UltimateCharacterController/Scripts/Character/UltimateCharacterLocomotion.cs:331)

I've undone my changes and run through the documentation pdf, but can't really get a grip on this.

Does anybody know where I should begin to debug?
This seems related to the one of the common errors listed on the integration pages under the Common Errors & Mistakes section

Not sure how or why it broke/got solved on its own. But perhaps reading through the that paragraph will give you some idea