I have downloaded the newer version of the plugin and my players just experienced this today, the plugin has been fine and they can still sell things to the store, they can't buy commands or items. 

This is an error from buying an item: 

Failed to call hook 'cmdBuyItem' on plugin 'ServerRewards v0.4.70' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.ServerRewards.cmdBuyItem (ConsoleSystem+Arg arg) [0x0005d] in :0
  at Oxide.Plugins.ServerRewards.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00f8e] in :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

This is what's in my oxide compiler log:
[SERVER v1.0.24] Started as service
[SERVER v1.0.24] Running as service
[SERVER v1.0.24] Got Message: Ready
[SERVER v1.0.24] Got Message: Compile
[SERVER v1.0.24] Console: ServerRewards.cs(417,24): warning CS0219: The variable `categories' is assigned but its value is never used
ServerRewards.cs(40,29): warning CS0649: Field `Oxide.Plugins.ServerRewards.uiFadeIn' is never assigned to, and will always have its default value `false'
ServerRewards.cs(26,33): warning CS0169: The private field `Oxide.Plugins.ServerRewards.EventManager' is never used
ServerRewards.cs(26,47): warning CS0169: The private field `Oxide.Plugins.ServerRewards.HumanNPC' is never used
ServerRewards.cs(26,67): warning CS0169: The private field `Oxide.Plugins.ServerRewards.PlaytimeTracker' is never used

The compiler log will not show any usage errors, only any errors or warnings when compiling the plugin.

Failed to call hook 'cmdBuyItem' on plugin 'ServerRewards v0.4.71' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.ServerRewards.cmdBuyItem (ConsoleSystem+Arg arg) [0x0005d] in <572483f18b06484da0f437e93f282ae1>:0
  at Oxide.Plugins.ServerRewards.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00e58] in <572483f18b06484da0f437e93f282ae1>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <008b12f41ec4452da1a5497eeb849299>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <ac41dd3599754d448b8c218b34645820>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <ac41dd3599754d448b8c218b34645820>:0

Failed to call hook 'cmdBuyItem' on plugin 'ServerRewards v0.4.73' (NullReferenceException: Object reference not set to an instance of an object)
at Oxide.Plugins.ServerRewards.cmdBuyItem (ConsoleSystem+Arg arg) [0x0005d] in <bfec9aa9ddc34ea48824f67bba3296ea>:0
at Oxide.Plugins.ServerRewards.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00e58] in <bfec9aa9ddc34ea48824f67bba3296ea>:0
at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <d09a1f46ca2f4432811bcfe45ad13c7b>:0
at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <cf88a28c7fb44d36890d85a78331cc9d>:0
at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <cf88a28c7fb44d36890d85a78331cc9d>:0

I fixed it by removing a corrupted CooldownData file that only contained "null" inside... (unload, delete, reload) works now.