I just got this bug. It happened during lag spike on Shadowdale. I was moving items from character to shared bank, lag started and one item I moved disappeared from character bank, but did not appear on shared bank.
It looks like there is some "cache" space that got filled by the transfer that did not clear. Item got stuck in there and it does not move anymore. I get the message "You do not have enough space in your backpack to withdraw that", even after logout and game restart.
This is a serious problem. Because this can cause item loss.