Failed to call hook 'CCmdRefresh' on plugin '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 <ab9a7b8145ed434599f21bf407e61f6e>:0
                                                   1kb/s in, 12kb/s out
  at Oxide.Plugins.BackPumpJack.UpdateAndRefill (MiningQuarry miningQuarry) [0x00131] in <ab9a7b8145ed434599f21bf407e61f6e>:0
  at Oxide.Plugins.BackPumpJack.RefillMiningQuarries (System.Boolean AddMissing) [0x00029] in <ab9a7b8145ed434599f21bf407e61f6e>:0
  at Oxide.Plugins.BackPumpJack.CCmdRefresh (Oxide.Core.Libraries.Covalence.IPlayer iplayer, System.String command, System.String[] args) [0x0000c] in <ab9a7b8145ed434599f21bf407e61f6e>:0
  at Oxide.Plugins.BackPumpJack.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x006af] in <ab9a7b8145ed434599f21bf407e61f6e>: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 <beb2b64691c64e2b95b9949

Failed to run a 6.04 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 <ab9a7b8145ed434599f21bf407e61f6e>:0
                                                   1kb/s in, 13kb/s out
  at Oxide.Plugins.BackPumpJack.UpdateAndRefill (MiningQuarry miningQuarry) [0x00131] in <ab9a7b8145ed434599f21bf407e61f6e>:0
  at Oxide.Plugins.BackPumpJack.RefillMiningQuarries (System.Boolean AddMissing) [0x00029] in <ab9a7b8145ed434599f21bf407e61f6e>:0
  at Oxide.Plugins.BackPumpJack.<OnServerSave>b__28_0 () [0x00000] in <ab9a7b8145ed434599f21bf407e61f6e>:0

Can you send your config file  BackPumpJack.json

{
"Apply patch to user quarry to fix long run time": true,
"Apply patch for mining rates for more precise pM config params": true,
"Patch for ladder flyhack": true,
"Patch for light signal when quarry is running": true,
"Maximum stack size for diesel engine (-1 to disable function)": -1,
"Number of slots for diesel storage (-1 to disable function)": -1,
"Number of slots for output storage (-1 to disable function)": -1,
"Enable static quarry resource modifier": false,
"refill command name": "backpumpjack.refill",
"Info command name": "backpumpjack.info",
"Settings": {
"Use Teams": true,
"Use Friends": true,
"Use Clans": false,
"Use clan table": false,
"Block damage another player's survey crater": true,
"Block deploy a quarry on another player's survey crater": true
},
"Chat Settings": {
"Chat Prefix": "<color=#00FFFF>[BackPumpJack]</color>: ",
"Chat SteamID Icon": 0
},
"Permission List": [
{
"Permission": "backpumpjack.use",
"Priority": 0,
"Oil Crater Chance": 20.0,
"Oil Crater Settings": {
"Minimum Mineral Amount": 1,
"Maximum Mineral Amount": 1,
"Allow Duplication Of Mineral Item": false,
"Mineral Items": [
{
"Mineral Item Short Name": "crude.oil",
"Chance": 50.0,
"Minimum pM": 280.8,
"Maximum pM": 280.8
},
{
"Mineral Item Short Name": "diesel_barrel",
"Chance": 50.0,
"Minimum pM": 1.0,
"Maximum pM": 1.0
}
]
},
"Normal Crater Settings": {
"Modify Chance (If not modified, use default mineral)": 10.0,
"Minimum Mineral Amount": 1,
"Maximum Mineral Amount": 2,
"Allow Duplication Of Mineral Item": false,
"Mineral Items": []
}
},
{
"Permission": "backpumpjack.vip",
"Priority": 1,
"Oil Crater Chance": 40.0,
"Oil Crater Settings": {
"Minimum Mineral Amount": 2,
"Maximum Mineral Amount": 2,
"Allow Duplication Of Mineral Item": false,
"Mineral Items": [
{
"Mineral Item Short Name": "crude.oil",
"Chance": 50.0,
"Minimum pM": 38.0,
"Maximum pM": 338.0
},
{
"Mineral Item Short Name": "diesel_barrel",
"Chance": 50.0,
"Minimum pM": 1.0,
"Maximum pM": 1.5
}
]
},
"Normal Crater Settings": {
"Modify Chance (If not modified, use default mineral)": 50.0,
"Minimum Mineral Amount": 1,
"Maximum Mineral Amount": 3,
"Allow Duplication Of Mineral Item": false,
"Mineral Items": [
{
"Mineral Item Short Name": "stones",
"Chance": 50.0,
"Minimum pM": 1200.0,
"Maximum pM": 1200.0
},
{
"Mineral Item Short Name": "metal.ore",
"Chance": 50.0,
"Minimum pM": 150.0,
"Maximum pM": 150.0
},
{
"Mineral Item Short Name": "sulfur.ore",
"Chance": 40.0,
"Minimum pM": 120.0,
"Maximum pM": 120.0
},
{
"Mineral Item Short Name": "hq.metal.ore",
"Chance": 40.0,
"Minimum pM": 55.0,
"Maximum pM": 55.5
}
]
}
}
],
"Static quarry settings": {
"Stone": {
"Mineral Items": [
{
"Mineral Item Short Name": "stones",
"Resource per minutes (pM)": 2500.0
},
{
"Mineral Item Short Name": "metal.ore",
"Resource per minutes (pM)": 500.0
}
]
},
"HQM": {
"Mineral Items": [
{
"Mineral Item Short Name": "hq.metal.ore",
"Resource per minutes (pM)": 45.0
}
]
},
"Sulfur": {
"Mineral Items": [
{
"Mineral Item Short Name": "sulfur.ore",
"Resource per minutes (pM)": 150.0
}
]
},
"Oil": {
"Mineral Items": [
{
"Mineral Item Short Name": "crude.oil",
"Resource per minutes (pM)": 30.0
},
{
"Mineral Item Short Name": "lowgradefuel",
"Resource per minutes (pM)": 85.0
}
]
}
},
"Version": {
"Major": 1,
"Minor": 4,
"Patch": 23
}
}

One of the Mineral items definition is empty and cause the error. 
"Mineral Items": []

I will add more warning message in the plugin to detect config issue.

oh geez, how did I miss that. I have not altered the config in some time so figured it just stopped working.