Yesterday iv got the msg that my server-provider will update. As soon as the server went back on with the latest rust-version iv got this error whenever the server saves:

"

[Error] Failed to call hook 'OnServerSave' on plugin 'Backpacks v3.4.0' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.Backpacks.OnServerSave () [0x00000] in <e3788b26994d4e79bb9c1c1a4aeac975>:0

  at Oxide.Plugins.Backpacks.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x000db] in <e3788b26994d4e79bb9c1c1a4aeac975>:0

  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <31122a27a2414cd799150f8677cf39d4>:0

  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0

  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0"

Any idea what causes this?

The most likely cause is data corruption of the oxide/data/Backpacks.json file. Please check if it is intact.

The error message above explicitly states that you are running v3.4.0, which is not the latest version of the plugin. Corruption issues are automatically mitigated in v3.5.0 and newer.

Unfortunately, due to where this error is occurring, it means that Backpacks are probably not saving. This means that updating the plugin or reloading it will revert player Backpacks to a previous state. If you want to avoid that, your best bet is to hold off until the next wipe before updating, and simply ignore the errors for now. If you don't wipe Backpacks on wipe, then I suggest you update the plugin immediately.

thanks, didnt realize the version was outdated.