I am writing a blog where I describe a problem as leading to "an unfathomable chaos", which is being underlined by Grammarly plugin for Chrome. Is it a mistake on my behalf or Grammarly is getting it wrong?
It appears that Grammarly objects to the use of the word "chaos" as a countable noun, with a singular article "an". Indeed "chaos" is normally uncountable.
It is possible to use chaos as a countable noun. So "an unfathomable chaos" is perfectly good English, and Grammarly is indeed getting it wrong. But perhaps you should consider rephrasing. It's not very clear how a problem could be "a chaos".