It is Librarian Book Manager. From the comments there:
Code:
I was able to get over it by commenting out line 492 (adding -- at the beginning) of librarian.lua
if self.settings.alertEnabled then
--CENTER_SCREEN_ANNOUNCE:AddMessage(EVENT_SKILL_RANK_UPDATE, CSA_EVENT_LARGE_TEXT, SOUNDS.BOOK_ACQUIRED, GetString(SI_LIBRARIAN_NEW_BOOK_FOUND))
end
This will prevent the error from happening, but it will suppress the on-screen message of a new book. The chat message will be displayed.
Hope this fixes it for you.