Failed to call hook 'OnEntityDeath' on plugin 'Backpacks v3.0.2' (InvalidOperationException: Collection was modified; enumeration operation may not execute.)

This is the code that is failing:

                foreach (var item in _itemContainer.itemList)
                {
                    item.Remove();
                    item.DoRemove();
                }

I had this same problem on a plugin I made. Basically, the error is saying that your trying to iterate through a data structure and change that data structure at the same time. Once you change it (item.remove()), you can no longer continue enumeration. 

The solution is to iterate through _itemContainer.itemList.ToList() instead. Then, search for the item to remove that matches some unique key in your data structure. This is how I did it in a similar block of code:

                        foreach (var pl in pd.playerLocks.ToList())
                        {
                            if (pl.status == "InProgress" || pl.status == "InProgress2")
                            {
                                var plUpdate = pd.playerLocks.Single(r => r.lockID == pl.lockID);
                                plUpdate.status = "Cooldown";
                                plUpdate.expTime = DateTime.Now.AddSeconds(config.BypassFailureCooldownTime).ToString();
                            }
                        }

I forgot to mention that what is happening in game is that only the first item in the backpack is getting deleted.