I was surprised at the amount of guildmates that were unaware of this bug so I'm posting it here, hope it's not a repost
After lockpicking any kind of box, instead of opening just relog. You will be able to pick it again even if it says it's unlocked already. My suggestion is to save every green lockbox at the bank and start lockpicking them all when you have like 10 or 15 because it will reduce alot the number of times you have to relog. Also green lockboxes because they go green later than pickpocketed boxes.
So good luck and good lockpick