When I try to open any of the inventory with a command (ender, normal, my own, someone else's).
There's nothing there. However, for example, if I use the native command from EssentialsX - /invsee PLAYER. Then the player's inventory will really show me.
But EssentialsX doesn't suit me, because it doesn't know how to view offline players' inventories.

I don't know, but maybe it has something to do with Java 11. Because when the server core was running on Java 8, there were no problems with the plugin.

OpenInv version is 3.3.5.
Spigot version is "This server is running CraftBukkit version git-Spigot-4bd94dc-3a482bd (MC: 1.12.2) (Implementing API version 1.12.2-R0.1-SNAPSHOT)"
Log: https://pastebin.com/My3mbGBG

See lishid#173.