Unofficial Fallout 4 Patch

Looks as if this was intended. After you get past a certain point in the dungeon, that door will be locked if you leave the cell. Apparently, the quest designers assumed that the player would go for killing Kellogg (and then the elevator would be functional) and did not consider the possibility that someone could have his priorities on junk collecting.

Anyway, this can be fixed somehow, but this would still leave you with a problem because quest fixes won't take effect in a game where the affected quest is already running. Apparently though, this is the case with all your available saves.

I just encountered this issue for the same reason. You don't even have to go through that door, or even see that door. I think all you have to do is enter the area and exit again. As designed is game breaking. Google found me that a functioning workaround is just to go to the console and type "tcl" (which is no clip) float through the door and then do "tcl" again. This seems to unbreak things. However, this bug should get fixed, it's game breaking. I can't believe it hasn't been in 6 years. Maybe just lock that other door behind you so you can't use that either...

The specific chain of events that lead to this is:

1. The player enters a trigger box in FortHagen01 that sets stage 101 of MQ106.
2. Stage 101 attempts to open the door, but cannot because the door is unloaded. UFO4P_ShouldActivateOnNextLoad gets set on the door as an attempt to force it to open.
3. The player enters FortHagen02, causing the door to load.
4. The door successfully runs its OnLoad script event and updates to match its default open state.
5. For unknown reasons, the OnLoad script event does not appropriately process the UFO4P_ShouldActivateOnNextLoad property, leaving it set.
6. The player leaves FortHagen02, allowing the door to unload. They can do other things in this step too, such as simply leaving the fort and returning later.
7. The player returns to FortHagen02, which causes the door to load.
8. The door runs its OnLoad event, which properly processes UFO4P_ShouldActivateOnNextLoad and activates the door, which closes it despite the intended state of the original call being to open the door.
9. The player is now locked out of the Fort Hagen Command Center and is unable to proceed with Reunions without the use of console or a mod.

This is related to issue 33695, which is caused by UFO4P_ShouldActivateOnNextLoad being resolved immediately after the gate is closed by MQ106 stage 105, which is set by the trigger box that unlocks the final door into Kellogg's chamber and initiates the boss dialogue. It is also related to issue 24039, which appears to have been a failsafe to remove the door in case the player immediately backtracks into Fort Hagen. The door would otherwise be unlocked again by stage 2000 of MQ106, which is set by holding the post-reunions conversation with Piper and Nick Valentine.

Ah, right. I forgot to mention my suggested fix. The most appropriate fix would be to revert the unofficial patch's changes to MQ106, stage 101. This will cause the script to always call the SetOpen function, which is overwritten by Default2StateActivator. That function, with the unofficial patch installed, will abort processing due to the door being unloaded and will set UFO4P_ShouldActivateOnNextLoad only if the desired open/closed status of the door does not currently match its IsOpen status.

In the case of this door in Fort Hagen, that would leave UFO4P_ShouldActivateOnNextLoad set to false since the door has IsOpen set to true. In the event that another mod change's the open/closed status of the door, that would allow the trigger box to force it open as intended.