not sure how this is showing or even what caused it 

Failed to call hook 'cmdReply' on plugin 'PrivateMessages v0.4.0' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Game.Rust.Libraries.Player.Message (BasePlayer player, System.String message, System.String prefix, System.UInt64 userId, System.Object[] args) [0x00038] in <47e054d89cd54d7f97a78981ec32a70f>:0
  at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Message (System.String message, System.String prefix, System.Object[] args) [0x00000] in <47e054d89cd54d7f97a78981ec32a70f>:0
  at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Message (System.String message) [0x00008] in <47e054d89cd54d7f97a78981ec32a70f>:0
  at Oxide.Plugins.PrivateMessages+PrivateMessage.SendPM (Oxide.Core.Libraries.Covalence.IPlayer sender, Oxide.Core.Libraries.Covalence.IPlayer target, System.String message) [0x0012c] in <0e86eeb0db054275ac571bb722324ba3>:0
  at Oxide.Plugins.PrivateMessages.cmdReply (Oxide.Core.Libraries.Covalence.IPlayer player, System.String cmd, System.String[] args) [0x00071] in <0e86eeb0db054275ac571bb722324ba3>:0
  at Oxide.Plugins.PrivateMessages.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x001c6] in <0e86eeb0db054275ac571bb722324ba3>: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

Getting the same one running the current version from here on Rust.

Failed to call hook 'cmdPM' on plugin 'PrivateMessages v0.4.0' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Game.Rust.Libraries.Player.Message (BasePlayer player, System.String message, System.String prefix, System.UInt64 userId, System.Object[] args) [0x00038] in <3cd1f1cff4f041e0b42b8bcdd6b47519>:0 
  at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Message (System.String message, System.String prefix, System.Object[] args) [0x00000] in <3cd1f1cff4f041e0b42b8bcdd6b47519>:0 
  at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Message (System.String message) [0x00008] in <3cd1f1cff4f041e0b42b8bcdd6b47519>:0 
  at Oxide.Plugins.PrivateMessages+PrivateMessage.SendPM (Oxide.Core.Libraries.Covalence.IPlayer sender, Oxide.Core.Libraries.Covalence.IPlayer target, System.String message) [0x0012c] in <c0474865888e44659527526e47736372>:0 
  at Oxide.Plugins.PrivateMessages.cmdPM (Oxide.Core.Libraries.Covalence.IPlayer player, System.String cmd, System.String[] args) [0x00070] in <c0474865888e44659527526e47736372>:0 
  at Oxide.Plugins.PrivateMessages.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00153] in <c0474865888e44659527526e47736372>: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​

This recently started happening -- I have tried removing conf/lang files with no success. 

Failed to call hook 'cmdPM' on plugin 'PrivateMessages v0.4.0' (NullReferenceException: Object reference not set to an instance of an object) at Oxide.Game.Rust.Libraries.Player.Message (BasePlayer player, System.String message, System.String prefix, System.UInt64 userId, System.Object[] args) [0x00038] in <1fe1ece82fd64334b7a426da108021f6>:0 at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Message (System.String message, System.String prefix, System.Object[] args) [0x00000] in <1fe1ece82fd64334b7a426da108021f6>:0 at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Message (System.String message) [0x00008] in <1fe1ece82fd64334b7a426da108021f6>:0 at Oxide.Plugins.PrivateMessages+PrivateMessage.SendPM (Oxide.Core.Libraries.Covalence.IPlayer sender, Oxide.Core.Libraries.Covalence.IPlayer target, System.String message) [0x0012c] in <3b9d32cfc2234459b7412ab7a4767590>:0 at Oxide.Plugins.PrivateMessages.cmdPM (Oxide.Core.Libraries.Covalence.IPlayer player, System.String cmd, System.String[] args) [0x00070] in <3b9d32cfc2234459b7412ab7a4767590>:0 at Oxide.Plugins.PrivateMessages.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00153] in <3b9d32cfc2234459b7412ab7a4767590>: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 use this plugin as well, haven't see any errors.

I just randomly got the same error

[Logger] Direwolf05 (xxxxxxxxxxxxxxxx) ran command: pm magisk where his base at Failed to call hook 'cmdPM' on plugin 'PrivateMessages v0.4.0' (NullReferenceException: Object reference not set to an instance of an object) at BasePlayer.SendConsoleCommand (System.String command, System.Object[] obj) [0x00000] in <53fd1ca1946a436989367b4548c3e2bf>:0
at Oxide.Game.Rust.Libraries.Player.Message (BasePlayer player, System.String message, System.String prefix, System.UInt64 userId, System.Object[] args) [0x00038] in :0
at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Message (System.String message, System.String prefix, System.Object[] args) [0x00000] in :0
at Oxide.Game.Rust.Libraries.Covalence.RustPlayer.Message (System.String message) [0x00008] in :0
at Oxide.Plugins.PrivateMessages+PrivateMessage.SendPM (Oxide.Core.Libraries.Covalence.IPlayer sender, Oxide.Core.Libraries.Covalence.IPlayer target, System.String message) [0x0012c] in <85d3adb63a72410eb3e1877ddd741f6f>:0
at Oxide.Plugins.PrivateMessages.cmdPM (Oxide.Core.Libraries.Covalence.IPlayer player, System.String cmd, System.String[] args) [0x00070] in <85d3adb63a72410eb3e1877ddd741f6f>:0
at Oxide.Plugins.PrivateMessages.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00153] in <85d3adb63a72410eb3e1877ddd741f6f>: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

This is fixed now.

awesome thanks alot