So there is this mystery called "War of the collectors" in Lunden for which I was supposed to get a key and steal a statue from "Maximilian". I couldn't manage to grab the key and saw it was a common problem so I used a solution I found online which consisted in breaking the backdoor. I managed to get the statue and completed the quest "Rivalry for the ages" for Octavius back at the Museum in our camp.
I thought I was safe... However now that I have completed the main story I want to finish all the mysteries to get 100 %, I found out that the mystery "War of the collectors" in Lunden is still there and the key shows up but cannot be grabbed. Maximilian is standing in front of the statue in his house with the front door unlocked and there is now no way to complete the mystery.
I am on the latest update and it says this issue was resolved in a patch (1.1.0 I think) but not in my case.