Failed to call hook 'SilCommand' on plugin 'SignArtist v1.1.5' (NullReferenceException: Object reference not set to an i
nstance of an object)
  at Oxide.Plugins.SignArtist.HasCooldown (BasePlayer player) [0x00006] in <d71367e819e84911a0a8f64efed1fb77>:0
  at Oxide.Plugins.SignArtist.SilCommand (Oxide.Core.Libraries.Covalence.IPlayer iplayer, System.String command, System.
String[] args) [0x0004c] in <d71367e819e84911a0a8f64efed1fb77>:0
  at Oxide.Plugins.SignArtist.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x0014d] in
 <d71367e819e84911a0a8f64efed1fb77>: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 <4452f821def6406d834
e4149849fe7ea>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <4452f821def6406d834e414
9849fe7ea>:0

Asking for help, what am I doing wrong? It used to work.

Just checking, do you have "Time in seconds between download requests (0 to disable)" set to something other than 0?
Also, which perms does this player have for SignArtist?

The problem was found. The contents of the config SignArtist.json was null. This is weird. Thanks.