Failed to Save player profile

type /save-all in your server console
Result:
[INFO] Saving...
[WARNING] Failed to save player data for calebdundas
[WARNING] Failed to save player data for shadowpat
[INFO] Saved the world
When you disconnect and then reconnect, the server forgets what you had in your inventory, and gives you what you had in a previous successful save! Also, you can use this glitch to duplicate items by logging into your server, putting all of your inventory items into a chest, logging out of the server, rejoining the server, and all of your items will be in your inventory AND the chest!!!! This one REALLY needs to be fixed!