I'm not sure what's causing this error, but it's definitely coming from AzeriteForge interacting with something else-- I can load every single addon except AzeriteForge and the error doesn't trigger. It happens as soon as I log in or reload UI.

The addon referenced in the last two lines can change-- it's come from GatherMate2, ArchaeologyHelper, AuctionLite, AngryAssignments... but otherwise it's the same lines in AceEvent that seem to be causing trouble. Possibly related to this event

Message: Interface\AddOns\ElvUI\Modules\skins\skins.lua:1322: attempt to index field 'private' (a nil value)
Time: Thu Sep 27 13:05:38 2018
Count: 1
Stack: Interface\AddOns\ElvUI\Modules\skins\skins.lua:1322: attempt to index field 'private' (a nil value)
Interface\AddOns\ElvUI\Modules\skins\skins.lua:1322: in function ?' ...te2\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:119: in function <...te2\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:119> [C]: ? ...te2\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:29: in function <...te2\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:25> ...te2\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:64: in function Fire'
...ddOns\GatherMate2\Libs\AceEvent-3.0\AceEvent-3.0.lua:120: in function <...ddOns\GatherMate2\Libs\AceEvent-3.0\AceEvent-3.0.lua:119>

Locals: