Failed to call hook 'OnServerInitialized' on plugin 'DiscordCore v2.0.3' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Ext.Discord.WebSockets.Socket.Disconnect (System.Boolean attemptReconnect, System.Boolean shouldResume, System.Boolean requested) [0x00071] in <cab1b2f84bde4043800dc62cde5a64a4>:0

  at Oxide.Ext.Discord.BotClient.DisconnectWebsocket (System.Boolean attemptReconnect, System.Boolean attemptResume) [0x0000c] in <cab1b2f84bde4043800dc62cde5a64a4>:0

  at Oxide.Ext.Discord.BotClient.AddClient (Oxide.Ext.Discord.DiscordClient client) [0x0014e] in <cab1b2f84bde4043800dc62cde5a64a4>:0

  at Oxide.Ext.Discord.BotClient.GetOrCreate (Oxide.Ext.Discord.DiscordClient client) [0x00054] in <cab1b2f84bde4043800dc62cde5a64a4>:0

  at Oxide.Ext.Discord.DiscordClient.Connect (Oxide.Ext.Discord.DiscordSettings settings) [0x000e6] in <cab1b2f84bde4043800dc62cde5a64a4>:0

  at Oxide.Plugins.DiscordCore.ConnectClient () [0x00027] in <27149d11f33a4711923c1f8d74235479>:0

  at Oxide.Plugins.DiscordCore.OnServerInitialized () [0x00043] in <27149d11f33a4711923c1f8d74235479>:0

  at Oxide.Plugins.DiscordCore.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x000d0] in <27149d11f33a4711923c1f8d74235479>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <31122a27a2414cd799150f8677cf39d4>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <0fbce81d88d64454b3e7abb24df7026b>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <0fbce81d88d64454b3e7abb24df7026b>:0

This is only happening on a restart. If after the restart I reload the plugin, it's fine

Merged post

Discord Clans by K1lly0u

This is the plugin causing conflict. If I remove it, it works. Also reported it his side. But weirdly discord core works just fine if I reload it AFTER server is fully restarted.

This looks to be more of an extension issue. It should be resolved in the next Discord Extension update.