During the mission “The Old Guard”, the gun supposed to be holding the doors closed is missing, and the doors to the storage are marked as “Locked”
The robots are inside, and are doing all too well as I can’t get to them.
Naturally, as the doors can’t be opened with the shotgun in question being missing, the mission can’t be completed.
I did this mission with a friend during a game he was hosting, and was able to complete it without problems. This bug then decided to show up during my solo-playthrough at a later point.