what could be the cause?

Failed to call hook 'OnEntityKill' on plugin 'C4Watch v1.1.1' (KeyNotFoundException: The given key '76561198984668410' was not present in the dictionary.)
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <e3aa8db986d7457ca73f10110f1283f5>:0 
  at Oxide.Plugins.C4Watch.OnEntityKill (TimedExplosive entity) [0x00056] in <f881e72b12fd4c84a60cf659d29b3669>:0 
  at Oxide.Plugins.C4Watch.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00266] in <f881e72b12fd4c84a60cf659d29b3669>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <795304323ac74a298b8ed190a1dfa739>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000de] in <beb2b64691c64e2b95b99491bd85442c>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <beb2b64691c64e2b95b99491bd85442c>:0 

I’ll take a look into this, thank you for posting! 

Changed Status from Pending to Work in Progress

I've identified some potential causes of this, and will be testing a patch soon. In the meantime, if you could send the state of your configuration file, I would appreciate the extra info. Thank you!

Too Easy mate 

C4Watch.json

Just fixed this in 1.1.2, it was occurring when a player attempted to detonate RF C4 after throwing it down, disconnecting from the server, and reconnecting.  Thank you for bringing this to my attention! 

Changed Status from Work in Progress to Closed

Changed Fixed In to 1.1.2