Trying to track this down and not having much luck. Maybe someone here can help?
After the Homestead update I started getting this error at random when double-clicking or dragging to delete items from my inventory. Right-click then selecting use item from the menu also causes it.
I deleted all saved variables including ZOS ingame settings, and disabled all addons, then started enabling one by one and it seemed that EMACS was causing it, which seemed unlikely since all it does is return true for the KEYBINDING_MANAGER:IsChordingAlwaysEnabled() function. But then after a few reloads it came back. Then it went away.
It doesn't actually seem related to any one addon, as far as I can tell.