An auxiliary language, in the sense of this fallacy, is a language invented for the purpose of international discourse. The fallacy part is merely that such languages exist, and that moreover they often try to be logical or philosophically rigorous. This conflicts with the way that languages actually work.
As nsh said, "communication is primarily aimed at conveyence of meaning, and less at universal truth". Languages trying to test out or utilise the Sapir-Whorf Hypothesis are usually egregious contraveners of this maxim.
Type: Anti Pattern, Linguistics Pattern