Basically, there was talk of N having another solo fan meeting (he's had a few before and has had one each year in Japan since debut) and some "fans" got pissed and were tweeting mean things at him.
“Let’s not make someone into a target of hate. Have you considered what it means to the other person when you draw your sword?” — Hongbin
Here are some translated tweets that "fans" sent:
“Not even the king would hold a birthday party for 10 years in the same country.”
“If you love all your fans, then have a birthday party with all of your fans.”
“Stop making excuses on why you have had a birthday party by yourself for 10 years. Everybody knows that something is off.”
Other fans agreed with Hongbin, defending N’s actions.
“How can you call yourselves Vixx fans?”
“Thank you Hongbin for gathering the courage to say something”
“Even if you are a fan of one person and not the whole group, that is the brother and teammate of someone special to you. How can you say that about them?”