As it turned out, this author posted a "cracked" version of the plugin from another well-known site (Direct_Leaks). I recommend using the same plugin from LegitSpigot, which, unlike Kate, does not expose plugins from other sites)
the plugin works on the first startup but once the server restarts we go back to the same problems
"[LeaderHeads] Could not start up the LeaderHeads core. Error code: 3"