This error is typically caused by add-ons in the game's User Interface (UI). In order to offer the greatest amount of flexibility and functionality for the player, the User Interface in World of Warcraft has been designed so that it may be customized with add-ons. However, UI customization is not a supported feature of the game. This error may occur even if add-ons have not been installed.
This error can easily be resolved by resetting World of Warcraft's User Interface.
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Article ID: 100778