Noticed names stopped syncing on discord after a while when the plugins is running, found these errors in the server logs:

Failed to call hook 'OnUserNameUpdated' on plugin 'DiscordSync v1.0.0' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.DiscordSync.GetGuildMember (System.String discordId) [0x00018] in <dc2c47864efc4fe495d71c63d7c6e063>:0 
  at Oxide.Plugins.DiscordSync.HandleNick (Oxide.Core.Libraries.Covalence.IPlayer player, System.String discordId) [0x0001d] in <dc2c47864efc4fe495d71c63d7c6e063>:0 
  at Oxide.Plugins.DiscordSync.OnUserNameUpdated (System.String id) [0x00014] in <dc2c47864efc4fe495d71c63d7c6e063>:0 
  at Oxide.Plugins.DiscordSync.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00147] in <dc2c47864efc4fe495d71c63d7c6e063>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <33f7c42a56934dae8ec04b11017dce31>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <b0a93b504f034a0db8b1c3d2503c0987>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <b0a93b504f034a0db8b1c3d2503c0987>:0 

Current fix is to reload the plugin when these errors start happening and name sync in discord works again.