Could not initialize plugin 'DiscordSignLogger v1.0.7' (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) [0x00079] in <4f4e3bb921964f96b3adc8eae27c197b>:0
at Oxide.Ext.Discord.WebSockets.Socket.Shutdown () [0x00000] in <4f4e3bb921964f96b3adc8eae27c197b>:0
at Oxide.Ext.Discord.BotClient.ShutdownBot () [0x00037] in <4f4e3bb921964f96b3adc8eae27c197b>:0
at Oxide.Ext.Discord.BotClient.RemoveClient (Oxide.Ext.Discord.DiscordClient client) [0x0002a] in <4f4e3bb921964f96b3adc8eae27c197b>:0
at Oxide.Ext.Discord.DiscordClient.Disconnect () [0x00028] in <4f4e3bb921964f96b3adc8eae27c197b>:0
at Oxide.Ext.Discord.DiscordClient.CloseClient (Oxide.Ext.Discord.DiscordClient client) [0x00004] in <4f4e3bb921964f96b3adc8eae27c197b>:0
at Oxide.Ext.Discord.DiscordClient.OnPluginRemoved (Oxide.Core.Plugins.Plugin plugin) [0x00015] in <4f4e3bb921964f96b3adc8eae27c197b>:0
at Oxide.Ext.Discord.DiscordClient.OnPluginAdded (Oxide.Core.Plugins.Plugin plugin) [0x00000] in <4f4e3bb921964f96b3adc8eae27c197b>:0
at Oxide.Core.Plugins.PluginManager.AddPlugin (Oxide.Core.Plugins.Plugin plugin) [0x00038] in <bae5f1223fce49c493b01571c99dce02>:0
at Oxide.Core.OxideMod.PluginLoaded (Oxide.Core.Plugins.Plugin plugin) [0x00035] in <bae5f1223fce49c493b01571c99dce02>:0

I have an almost similar problem.

Have you found a solution?

Added '// Reference: System.Drawing' in plugin 'DiscordSignLogger'
DiscordSignLogger was compiled successfully in 2446ms
Could not initialize plugin 'DiscordSignLogger v1.0.8' (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) [0x00092] in <04348bf9a35746d283d4a8b439245576>:0 
at Oxide.Ext.Discord.WebSockets.Socket.Shutdown () [0x00001] in <04348bf9a35746d283d4a8b439245576>:0 
at Oxide.Ext.Discord.BotClient.ShutdownBot () [0x0003a] in <04348bf9a35746d283d4a8b439245576>:0 
at Oxide.Ext.Discord.BotClient.RemoveClient (Oxide.Ext.Discord.DiscordClient client) [0x00032] in <04348bf9a35746d283d4a8b439245576>:0 
at Oxide.Ext.Discord.DiscordClient.Disconnect () [0x00029] in <04348bf9a35746d283d4a8b439245576>:0 
at Oxide.Ext.Discord.DiscordClient.CloseClient (Oxide.Ext.Discord.DiscordClient client) [0x0000f] in <04348bf9a35746d283d4a8b439245576>:0 
at Oxide.Ext.Discord.DiscordClient.OnPluginRemoved (Oxide.Core.Plugins.Plugin plugin) [0x0001d] in <04348bf9a35746d283d4a8b439245576>:0 
at Oxide.Ext.Discord.DiscordClient.OnPluginAdded (Oxide.Core.Plugins.Plugin plugin) [0x00001] in <04348bf9a35746d283d4a8b439245576>:0 
at Oxide.Core.Plugins.PluginManager.AddPlugin (Oxide.Core.Plugins.Plugin plugin) [0x00038] in <bae5f1223fce49c493b01571c99dce02>:0 
at Oxide.Core.OxideMod.PluginLoaded (Oxide.Core.Plugins.Plugin plugin) [0x00035] in <bae5f1223fce49c493b01571c99dce02>:0 
No previous version to rollback plugin: DiscordSignLogger​

I have restarted the server and the problem is solved !