Ok 4th try is the charm:
What you need to do is close the Istance Window BEFORE you arrive at the mine, another will appear once you are inside, with the title "Undeserved Relic".
After the quest is given to me, I approach the mine and an Instance Window appears.
But this is not the right instance, there are Judges close to the entrance and no barricade inside.
I tried to abort and restart the mission 3 times to no avail...