English is my native language; I speak a British dialect and the original post is in British English. "I've just xyz" is standard outside American English and omitting "I've" is extremely common. I thought I was clear about that. In fact it seems like you haven't quite paid attention to, or haven't understood, everything I've written, because I quite expressly said I'm not Dutch as well and you seem to think I'm Dutch.

Regarding the is/are thing: you could also say "a number of problems with the post, but one of them is", and "none" is a short way of saying "not one", hence "none is"; in fact you'd instinctively say "a number of... but not one of them is". I'd say "none are" though out of habit. Your "a number is plural" reasoning is overthinking it.