This happened to one of my characters as well, I suspect it might have been because I had the 'automatically loot boxes' option activated on that character in the Lazy Writ Crafter addon but at the same time I tried to open the box manually as the boxes were being looted, and after that it wouldn't open. Not sure if that's the actual reason, it might have been a coincidence, but pointing it out in case you're also using that setting and something similar might have happened..
on two separate characters, on two different days now, I have been left with event boxes that I cant open. I tried restarting, reloading, and before and after the patch today, still can not open these boxes.
Did anything fix it for you? Or are they still bugged?
You might want to check the number of the transmute crystals you have. If you have maximum, the boxes with transmutes wont' open.