Failed to run a 8.50 timer in 'BackPumpJack v1.4.23' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.BackPumpJack+MiningSettings.RefillResourceDeposit (ResourceDepositManager+ResourceDeposit deposit) [0x00072] in <9d9d762a8f5644f79a0cc0382f53673a>:0
                                                   3kb/s in, 39kb/s out
  at Oxide.Plugins.BackPumpJack.UpdateAndRefill (MiningQuarry miningQuarry) [0x00131] in <9d9d762a8f5644f79a0cc0382f53673a>:0
  at Oxide.Plugins.BackPumpJack.RefillMiningQuarries (System.Boolean AddMissing) [0x00029] in <9d9d762a8f5644f79a0cc0382f53673a>:0
  at Oxide.Plugins.BackPumpJack.<OnServerSave>b__28_0 () [0x00000] in <9d9d762a8f5644f79a0cc0382f53673a>:0
  at Oxide.Core.Libraries.Timer+TimerInstance.FireCallback () [0x00018] in <beb2b64691c64e2b95b99491bd85442c>:0

Do you see this error frequently ?

If an admin use console command  backpumpjack.refill  Do you also see this error ?