Without answering how much conductivity you had, how it caused the circuit to fail, what part of your circuit failed, and in what way, you haven't really narrowed it down to the flux, even if cleaning helped. There are a number of more likely root causes of the kind of issue you describe. I can make time for going into that if you're interested.

"No-clean water soluble" is news to me. Don't know if I'd trust that in a humid environment.