Can you check to see if the request is hitting your server at all? There are two reasons that error could occur.

1) nzb360 can't reach the server at all.
2) Sonarr is denying the request, due to a bad API key, authentication, etc.

Once you can figure out which step we're at, it will help to debug this.
wow, I never thought I would make this mistake but the api key was wrong. I copied and pasted, I'm sure. But I just checked and the key from the webinterface was different from what nzb360 was using. Used burp to intercept the request and noticed the 401's coming in. Asked in #sonarr for further advice.

All good now