Line i've narrowed it down to
final Trade charge = new Trade("Commandhome", plugin.getEssentials()); user.getTeleport().teleport(user.getHome("home"), charge, PlayerTeleportEvent.TeleportCause.COMMAND);
2nd line, as once I comment that line out, there is no issue.
error: http://pastebin.com/bUT50JW0

Using Spigot 1.11,

if it helps, here is where the error is thrown in Spigot code
https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/nms-patches/ItemStack.patch#64
Also, error only happens when the item is only 1, if it's > 1 then no error.

Closing as this doesn't appear to be directly caused by EssentialsX - if you've found any other hints, feel free to reopen the issue and report back.