I can't speak for math, but I know that a few centuries ago, certain musical notes were regarded as 'The Devil's Music':

https://en.wikipedia.org/wiki/Tritone#Historical_uses http://news.bbc.co.uk/2/hi/uk_news/magazine/4952646.stm

Well, both of those articles explain that this was just perceived as a dissonant interval which could be used a a musical representation of the Devil. The interval was likely avoided for essentially aesthetic musical reasons, and the Wikipedia article says that "...suggestions that singers were excommunicated or otherwise punished by the Church for invoking this interval are likely fanciful." Unfortunately the BBC article seems to be light on facts and heavy on stereotypes of medieval monks as ignorant religious fundamentalists.

Especially since under neoplatonic thought lots of things are considered more real because they lack physical existence.

I wonder if this is maybe a confused reference to the privation theory of evil? In Aristotelean scholastic theology evil is simply a lack of being (since goodness is one of the transcendental properties of being), and so there's a connection between nothingness and evil.

I think ignorance is probably a simpler explanation, but thanks for bringing that up, I am not well-versed in medieval theology.