I logged in after taking a two hour break and one of my items was missing. The server had just reset so that might have done it. This might be the second time it has happened to me. The first time I just dismissed as using the item since it was a single use item. Now I lost a mask of pitiless flames (the one you find in the marching dead quest) it had 6 charges when I logged off.
I am not asking to have the items replaced just to know why this is happening.
This is a quest item.
Some items that are important to quests, keys, maps or in this case - your helmet: are marked to disappear every reset.
This is not a bug, and will only happen to items that are important to quests and such.
WHen you login if you look in your combat window, you should see
[Questsys: Removing quest items]
Then Lost item: (Item name)
Etc.
Sorry!