Hi. When I start my server i get this http://pastebin.com/qJy84Abd

Thank you for bringing this to my attention. This is a confirmed bug. I have seen this recently when testing the dev builds on my server. I have no idea what is causing it because it is affecting ALL versions of itemjoin with the check for updates option. This error never occurred in the past so it must be something with craftbukkits website. I will continue to further investigate this.

This has been fixed as of ItemJoin v3.5 and it has been released!

You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=107086