The player complained about the loss of weapons from the backpack. I cannot confirm or deny his words. I don't have any information about moving items. Is it possible to add the movement of all backpack items to the log file?

A log file would cause too much performance loss.

If you can gather more info about which circumstances surrounded the player losing the weapon, I could brainstorm possible root causes, but as of now, there are no known bugs that could cause item loss, and there have not been any reports of item loss for at least a year, and there have been no significant changes in the past year which could have introduced item loss bugs.

• What specific item(s) did the player claim to lose?
  
  • How large was the stack?
  • On which page of the backpack was the item located (if using v3.9+)?
• How many distinct items did the player claim to lose?
• What time did the player last see the item?
• What time did the player notice the item was missing?
• How often / how many times has the player claimed this has happened?
• Does the player have permission to use the Fetch feature? If so, do they ever use that feature? If so, do they (or did they at the time) have any key binds to fetch the item that went missing? If so, the player may have accidentally fetched the item from the backpack, and then lost it another way.
• Had the player's backpack capacity changed since the time they saw the item and the time they noticed it was missing? If the backpack was over capacity for any reason, when the player opened their backpack, overflowing items would have been ejected into their inventory. If their inventory was full, the remaining items would be dropped in front of them. Such an event would also print in chat to the player to let them know that their backpack was overflowing. If the player did not realize this happened, they might have overlooked the item being in their inventory (which they could have then done something with), or they could have overlooked the item being dropped.
• Have you configured the server to drop backpacks on death? If so, did the player discover the item was missing when looting their dropped backpack, or simply between opening and closing the backpack while staying alive?
• Did the player disconnect and reconnect between the time the item was last seen and the time they noticed the item was missing?
• Was the item restricted according to the "blacklist" and/or "whitelist"? If so, was the player exempted by permission? If the player lost their exemption status (due to a permission change), or if the plugin was reconfigured to restrict that item since the player had place it into their backpack, when the player next opened their backpack, the restricted item would have been ejected to their inventory or dropped in front of them, with a chat message notifying them of the event. It's possible the player did not realize that happened and then overlooked the item on the ground, or lost/sold/stored the item another way after it was added to their inventory.
• What is your config setting for "Only Save Backpacks on Server-Save (true/false)"? Note that if you are using v3.9+, that option no longer exists (in case you are wondering why you no longer see it).