Thanks for taking the time to open a bug report!

Describe the bug
I have a disguise plugin that uses NickAPI to disguise the player. For some reason, when putting NickAPI on the server, ItemJoin seems to have issues giving items on join. Every join is different. For example, the first join could only give item 1 and item 2, but not item 3, and others sometimes give only item 2 and item 3, etc. There is never a case when all items are given. There aren't any errors for ItemJoin; however, NickAPI gets this strange error: https://pastebin.com/iXexr5h4

To Reproduce
Steps to reproduce the behavior:

1. Install NickAPI and use this config (https://pastebin.com/ncrb9fZ1)
2. Have ItemJoin on the server, with this config (https://pastebin.com/hfrvYecd) and these items (https://pastebin.com/r86FxUVn)
3. Join with a cleared inventory, clear inventory, rejoin and not all items will be given

Expected behavior
I expect the player to receive all items when joining using NickAPI. The player isn't even disguised when this happens either. It doesn't happen when the player is disguised with NickAPI.

Screenshots
Expected inventory: https://i.imgur.com/jk7OxFV.png
Some inventories with NickAPI installed:

• https://i.imgur.com/ZreKQ6P.png
• https://i.imgur.com/TWjTTuV.png

Pastebin
Config: https://pastebin.com/hfrvYecd
Items: https://pastebin.com/r86FxUVn

Please complete the following information:

• Resource Version and Build Number: v5.22-RELEASE-b734
• Server Version: mSpigot 1.8.8

Additional context
N/A

Okay so, this seems to simply be a feature compatibility issue.
I have added support for NickAPI so it now checks their database of players and fetches the proper player information.
Link; https://ci.craftationgaming.com/job/ItemJoin/764/
Remember to remove your old ItemJoin.jar file
Let me know!~

This feature has been officially added in the latest release.

You can now download it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=431799