I noticed this in my log

13:19 [Error] Failed to call hook 'OnEntityDeath' on plugin 'PlayerChallenges v2.0.47' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.PlayerChallenges.OnEntityDeath (BaseCombatEntity entity, HitInfo info) [0x003cb] in <30bc6ce5099f4fe28ed19c7340db7e51>:0 
  at Oxide.Plugins.PlayerChallenges.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00e88] in <30bc6ce5099f4fe28ed19c7340db7e51>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <31122a27a2414cd799150f8677cf39d4>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <0fbce81d88d64454b3e7abb24df7026b>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <0fbce81d88d64454b3e7abb24df7026b>:0 ​

I got the same now. 

I'll see if it's gone after I wipe and clear out the data folder.

This is likely from an invalid/corrupt config file, but unsure.