It was with:
* Unity 2021.1.12f1 to 2021.1.15f1 (I probably started with an earlier version of Unity, but I try to update them when they come out)
* Third Person Controller version 2.3.4
* Agility Pack 1.1.1
* Climbing Pack 1.0.1
* The last version of the Ultimate Inventory System (and...
I encountered this too. I don't know how I got into this state, but I was able to fix it while investigating the system. I think the reorderable list got corrupted at some point and was using bad values in the Item Set Manager, Items List.
I think a combination of things fixed it for me, but...