I turn on debug to see what error this plugin might have. Here is one on startup.
[19:34:29] [Server thread/INFO]: [ItemJoin] Enabling ItemJoin v5.0.3
https://pastebin.com/n5wzYrap

I am 99.9% sure that this is resolved in the latest snapshot; http://ci.craftationgaming.com/job/ItemJoin/255/

Let me know!

EDIT: Closed due to inactivity, if this didn't resolve your problem please reopen this issue.

I will report if this issue is not fix.