Failed to call hook 'OnServerSave' on plugin 'PlayerChallenges v2.0.45' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.PlayerChallenges.SaveData () [0x00000] in <a73f09570ac7419aab491beed830e33d>:0 
  at Oxide.Plugins.PlayerChallenges.OnServerSave () [0x00000] in <a73f09570ac7419aab491beed830e33d>:0 
  at Oxide.Plugins.PlayerChallenges.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00acc] in <a73f09570ac7419aab491beed830e33d>: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

Sounds like your oxide/data for the plugin may be corrupt.

hmm, is there any way to search for it? besides this error, i also had another error that failed with the saving of the IO ( rustedit ) connections, but i dont have any custom powerlines created.... yet :P 

I'd just open the file and look to see if it's incomplete or corrupt.