I get the same error, even after manually unloading and loading the plugin.
It seems like it's able to get every other message, but most of the messages fail with this error:
(08:51:08) | Failed to call hook 'OnPlayerChat' on plugin 'Rustcord v1.1.1' (InvalidOperationException: Collection was modified; enumeration operation may not execute.)
at System.ThrowHelper.ThrowInvalidOperationException (System.ExceptionResource resource) [0x0000b] in <e1a80661d61443feb3dbdaac88eeb776>:0
at System.Collections.Generic.List`1[T].ForEach (System.Action`1[T] action) [0x00053] in <e1a80661d61443feb3dbdaac88eeb776>:0
at Oxide.Ext.Discord.REST.RESTHandler.BucketRequest (Oxide.Ext.Discord.REST.Request request, System.Action`1[T] callback) [0x0000e] in <c70faa7e50854fe0b707ce4ec3a240ad>:0
at Oxide.Ext.Discord.REST.RESTHandler.CreateRequest (Oxide.Ext.Discord.REST.RequestMethod method, System.String url, System.Collections.Generic.Dictionary`2[TKey,TValue] headers, System.Object data, System.Action`1[T] callback) [0x0007b] in <c70faa7e50854fe0b707ce4ec3a240ad>:0
at Oxide.Ext.Discord.REST.RESTHandler.DoRequest[T] (System.String url, Oxide.Ext.Discord.REST.RequestMethod method, System.Object data, System.Action`1[T] callback) [0x0000f] in <c70faa7e50854fe0b707ce4ec3a240ad>:0
at Oxide.Ext.Discord.DiscordObjects.Channel.GetChannel (Oxide.Ext.Discord.DiscordClient client, System.String channelID, System.Action`1[T] callback) [0x0001f] in <c70faa7e50854fe0b707ce4ec3a240ad>:0
at Oxide.Plugins.Rustcord.OnPlayerChat (ConsoleSystem+Arg arg) [0x0003c] in <c72b13c6455b4b338407bb2096617afd>:0
at Oxide.Plugins.Rustcord.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x0008d] in <c72b13c6455b4b338407bb2096617afd>:0
at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <9affce1cd15c4ec183941adef8db1722>:0
at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <4452f821def6406d834e4149849fe7ea>:0
at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <4452f821def6406d834e4149849fe7ea>:0
I am using the correct channel id, because it works sometimes:

I also see this error message when loading the plugin:
(08:54:09) | Unloaded plugin Rustcord v1.1.1 by Kirollos & OuTSMoKE
(08:54:14) | Loaded plugin Rustcord v1.1.1 by Kirollos & OuTSMoKE
(08:54:15) | Failed to run a 1.00 timer in 'Rustcord v1.1.1' (NullReferenceException: Object reference not set to an instance of an object)
at Oxide.Plugins.Rustcord.<Discord_Ready>m__1 () [0x00000] in <c72b13c6455b4b338407bb2096617afd>:0
at Oxide.Core.Libraries.Timer+TimerInstance.FireCallback () [0x00018] in <4452f821def6406d834e4149849fe7ea>:0