Does anyone know what the correct plugin version is for Spigot 1.8.8? The latest one seems to be buggy.

All versions of ItemJoin support 1.8.8 so if there is a bug a proper bug report would be recommended so I can solve the issue. As well as including your /itemjoin dump information.

https://pastebin.com/CRNp5wvQ

When I use /itemjoin reload, this comes up.

https://pastebin.com/CxhT0Hbw
What happens is that of all those items that appear, only two load for me, and when I use /itemjoin menu, the menu does not work. I click on them and they do not perform their functions.

Going to close this since it seems this has been now answered in your Discord Ticket.
If you are unaware please see ticket-1345 on our Discord Server.