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Please describe your question or what you need help with, in the best detail you can.
When joining and receiving items an error appears.

Pastebin
Config: https://pastebin.com/7FG5ARvP
En-lang: https://pastebin.com/iEwB9YC2
items: https://pastebin.com/A0yaNedi

Please complete the following information:

• Resource Version and Build Number: Latest
• Server Version1 SPigot 1.16.3

Additional context
https://pastebin.com/3bPxXUer

This issue happens when a player joins and recieves items.

This bug has been officially fixed in the release of ItemJoin v5.2.0.
You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=383801

Please read the changelog as the documentation has not been updated yet;
https://github.com/RockinChaos/ItemJoin/wiki/Recent-Changes

Should already be resolved in the latest snapshot; https://ci.craftationgaming.com/job/ItemJoin/574/

agreed!