For some reason all Backpack Items are being deleted whenever a player logs out then back in even after a server save etc. I have the most up to date version of the plugin.

When did this start happening? Can you paste your configuration?

This started happening yesterday after the recent oxide/rust update.

{
  "Drop on Death (true/false)": true,
  "Erase on Death (true/false)": false,
  "Use Blacklist (true/false)": false,
  "Clear Backpacks on Map-Wipe (true/false)": false,
  "Only Save Backpacks on Server-Save (true/false)": false,
  "Blacklisted Items (Item Shortnames)": ["autoturret", "lmg.m249"],
  "Backpack Size (1-7 Rows)": 7
}

I just tested with that configuration using the latest version of Rust and Oxide, and I'm unable to reproduce the issue.

Given that configuration, I recommend the following troubleshooting steps.

1. Open your backpack, place some items in it, and close it.
2. Check the oxide/data/Backpacks/{steamid}.json file on the server. It should be updated with a representation of the items (id, position, amount, etc.). If the file is not there or doesn't roughly match what you put in it, something went wrong.
3. Disconnect from the server.
4. Check the file again to see if it is the same as before. If it has changed, such as emptied out, something went wrong.
5. Reconnect to the server.
6. Check the file again. If it has changed, something went wrong.
7. Open your backpack. If your items are gone, something obviously went wrong, but don't close the backpack yet. Go check the file again.
8. Close your backpack. If it was visibly empty in the previous step, this will probably update the file to empty.