Hi, 
One of my players started the sulfur quarry, quarry started and consumed the first barril. He came back after an hour or so but only one baril was "consumed" and when he tried to access the loot container he couldn't, no interaction possible. When unloading PublicQuarryLock the loot container was accessible again (with no loot, but because only the first baril was consumed I guess this is normal). But I got this message in console (the quarry at that location is indeed the sulfur quarry) : 

Failed to call hook 'OnQuarryToggledOff' on plugin 'PublicQuarryLock v1.0.32' (NullReferenceException: )
  at (wrapper managed-to-native) UnityEngine.Component.GetComponentFastPath(UnityEngine.Component,System.Type,intptr)
  at UnityEngine.Component.GetComponent[T] () [0x00021] in :0 
  at Oxide.Plugins.PublicQuarryLock.OnQuarryToggledOff (MiningQuarry quarry, System.Boolean enableMsg) [0x00147] in :0 
  at Oxide.Plugins.PublicQuarryLock.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x0088c] in :0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in :0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000de] in :0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in :0 

I have seen that in the past, sometimes, the hopper or fuel storage get deleted.
not sure why this is happening (maybe explosion damage), but the plugin detect the missing componenent and try to correct the situation when it reload.
the other NullReferenceException, is also because the hopper was deleted.
The quarry should work after a server restart ( or a plugin reload )

Thanks for your answer, indeed reloading the plugin fixed the problem. First time seeing this so I assume it doesn't happen a lot so all is wel!