Hello!
I have a lootbox that isn’t openable in Overwatch. I have the lootbox counter in the main menu stuck 1 above the lootboxes in the lootbox menu.
Like if I have 2 lootboxes in the lootbox menu, I will have 3 lootbox in the main menu. It’s been a few days and I don’t know how to fix it nor how/why it happened.
I’m having the same issue! I earned a lootbox from twitch drops at the very end of last season, but I don’t think it was redeemed to my account until the start of season 16. So it just shows that I have a lootbox that cannot be opened.
EDIT: apparently this is a known issue, and the devs stated in the “Known Issues” thread that all boxes in this state will be automatically opened on May 6th!
Oh, thanks for the answer!
Understood, the phantom lootbox gets to live until then. 
Happened to me again today after earning a free lootbox from Twitch drops. It says I have a lootbox in the game waiting for me on the main menu screen but when I click on the lootbox screen it’s not actually there.
I had this happen as well from Twitch drops this week. I collected both the normal and the epic and it had me open my epic lootbox first. That opened just fine. Then it didn’t give me the option to open more lootboxes, so I exited to the main menu where it showed a counter of one unopened normal lootbox. When you click on it it takes you to the normal lootbox menu but there was no option to open the box. I exited out of the entire game, relaunched the game, and suddenly no lootboxes were available to open BUT when I checked the lootbox history it showed I had already opened it when I hadn’t. Could this please be looked into?
EDIT: It looks like they updated the Known Issues thread to include this issue. The auto-opening of the lootbox on next login is the current workaround.