@ubi-keo I believe I used an ability to break the door, though I can't remember if it was the Dive of the Valkyries or Incendiary Powder Trap. Pretty sure it was the latter.

And no worries, thanks!

@ubi-keo Yes, it seems like some odd people have come across this issue when attempting to complete the "A Rivalry for the Ages" quest, where they're unable to retrieve the key or speak to Maximillian. It was suggested to break through the back door by chucking those fire pots, or using one of the special attack skills to break it to retrieve the item. A little time after, Max appeared with interact-able dialogue.

My Ubisoft account username is ennam123. Note, my ubisoft account seems to be linked to a very old email address of mine. I no longer has access to this email, so I am not sure on how to change this.

This is the last mystery left, which is stopping me from a platinum!!

A while back, I was unable to retrieve the key for this event. When I came across the A Rivalry for the Ages” quest, I ended up breaking into the room via the backdoor to retrieve the item. Now Maximilian has reappeared and I offered to pay her to get into the locked room. However, because the item has already been taken, it's stuck and nothing happens.

Please advise/fix as I've spent 170hrs on a game for platinum just to be stuck because one bug!

I have uploaded a video of my issue: