User inventory cleared when disconnected

Some players inventory gets cleared when leaving the server, this problem is completely random and some users get it more often then others. while other players never even experience the bug at all.

Message from console:
[20:38:30] [Server thread/INFO]: Players: []Nexoness
[20:38:40] [Server thread/INFO]: Nexoness lost connection: Disconnected
[20:38:40] [Server thread/INFO]: Gathering player's inventory
[20:38:40] [Server thread/INFO]: Left × Nexoness
[20:38:40] [Craft Scheduler Thread - 632/INFO]: Checking player's inventory
[20:38:40] [Server thread/INFO]: ERROR in player's inventory
[20:38:40] [Server thread/INFO]: Will clear players inventory on next join

(I changed the name of the player to my own username)