It was in my garrison, with NPC nameplates only visible when they're selected. Selecting an NPC gave this error:

2x Kui_Nameplates\hook.lua:24: Attempt to access forbidden object from code tainted by an AddOn
[string "=[C]"]: in function HookScript' [string "@Kui_Nameplates\hook.lua"]:24: in function <Kui_Nameplates\hook.lua:21> [string "=[C]"]: in function AcquireUnitFrame'
[string "@Blizzard_NamePlates\Blizzard_NamePlates.lua"]:114: in function `OnNamePlateAdded'
[string "@Blizzard_NamePlates\Blizzard_NamePlates.lua"]:56: in function <...eBlizzard_NamePlates\Blizzard_NamePlates.lua:47>

6809f6f Thanks.