"Find a Missing Key" - Quest Bug

So I've had this issue for a while and it actually caused me to stop playing the game. A side quest called "Find a Missing Key", that I failed to complete, during the Wicked Hearts quest line, is now stuck in my active quest log, and is permanently and distractingly displayed in the top right corner. Since there is no way to go back to the Winter Palace and finish this quest, it just sits there, as a constant eyesore, for the rest of the game.

I tried to ignore it, but it eventually just soured me on the game altogether.

I see that a couple of people are having the same issue, and I'm a little baffled that it hasn't been fixed after 7 patches.

I guess it's a minor issue, in the grand scheme, but it's endlessly frustrating.