i believe its a one time error when the server starts and no, no one was on the server at that time.
14:09 [Info] Loaded plugin XPerience v1.2.8 by MACHIN3
14:11 [Error] Failed to call hook 'OnPlayerHealthChange' on plugin 'XPerience v1.2.8' (NullReferenceException: Object reference not set to an instance of an object)
at Oxide.Game.Rust.Cui.CuiHelper.DestroyUi (BasePlayer player, System.String elem) [0x00035] in <288c15f72eb2462c85dd40b43bf86fe3>:0
at Oxide.Plugins.XPerience.DestroyUi (BasePlayer player, System.String name) [0x00000] in <f680cd5e51b543ac9ff932ca61781019>:0
at Oxide.Plugins.XPerience.LiveStats (BasePlayer player, System.Boolean update) [0x00039] in <f680cd5e51b543ac9ff932ca61781019>:0
at Oxide.Plugins.XPerience.OnPlayerHealthChange (BasePlayer player, System.Single oldValue, System.Single newValue) [0x0001d] in <f680cd5e51b543ac9ff932ca61781019>:0
at Oxide.Plugins.XPerience.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x017b1] in <f680cd5e51b543ac9ff932ca61781019>:0
at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0
at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <12678b905a6d43c3a9cc366104306651>:0
at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <12678b905a6d43c3a9cc366104306651>:0
doesn't even show up after plugin reload, only on serverrestart. Not sure whats the problem there. Also we got another one like that called "Failed to call hook 'OnResearchCostDetermine' on Plugin XPerience v1.2.8'" not sure whats that about.
Maybe these are just some weird rust errors, i'm not so familiar with Rust modding and just wanted to report it, in case its something serious. If not, i'm sorry :D