Just loaded up a clean version of Bukkit 1.10.2 (I tried this with Spigot as well) ran it and joined the world with MC 1.10.2. I then put ItemJoin v3.8 into the plugins folder. The plugin loads fine with the server starting up. However, once I join, I get this error (attached). I don't have any custom worlds loaded, only the defaults that craftbukkit created. I join the servier just fine and the world opens in MC, but no items are in my inventory. I haven't modified the config.yml, en-lang.yml, or items.yml (but I plan to modified the items once I get this working). no other plugins are being used. Need some help if you can provide it. Thanks!

Good to hear appreciate the hard work. What version of Spigot would you recommend to avoid this for now?

This wont affect anything on your server. This is because spigot changed how they render maps and I have to find the new calculations for v3.9 so it no longer shows this message. Sadly its not an easy task but it will be fixed.

Thats the problem, they sprung this upon me and I havent been able to find when it changed. It must be a 1.11 update. But its no big deal its just a message that shows up in console, nothing actually happens in game. I think i might have finally located the new code for this.

ItemJoin v3.9 has been released and fixes this issue.
You can download it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=139837
If you have anymore problems feel free to open another bug report.