You would not lose anything, it would just be stuck in the bank. Unfortunately, an overflowing EJ bankbox cannot be used even to withdraw an item and fix the problem. The only solution is subscribing. But nothing is lost. Also of note is that, if you logged out while sitting on a loaded pack beetle, DO NOT dismount. EJ cannot ride a loaded beetle - you would have to unload the bug before logging out, or lose those items when it goes wild.