This looks to me like a personal vendetta

If anything, Andy would have a personal vendetta against the MSDN forum management for not backing him up. Since they did not, he resigned because policing Naomi's posts on the site would have been a full time job an he didn't have the time.

Why does the evidence under the circumstances not really concince me

I can't imagine how you do not find the evidence convincing. Or are you saying that Eric Den Doop is faking his logs and lying about the fact that "Andy" logged into foxite from the exact same IP address as Naomi uses (as shown in the screen shot).