I did not find a bug subforum for Kingmaker, so I thought I’d post about this here.
So the problem I was having is that despite checking all the requirements, I was unable to approach the Stag Lord Fort due to the book fog event.
Before anyone asks: I am on the latest patch (on GoG) on the enhanced edition and I checked all the chapter 1 requirements for clearing the fog (visited the Abandoned Hut, the Pine Patch, Nettle’s Crossong, killed Tartuccio, reported his death…).
Finally, I managed to solve the problem: what I had to do is to approach the Fort by following the river (as opposed to doing that from the east or the west roads). Specifically, I had to approach the Fort from the river’s left (western) bank. The other one did not work.
I am reporting this bug because some people I have talked to did not have to resort to this. Thus, I believe this is a bug. Hope the devs see this.
PS I am 100% sure I did all the quests I needed to unblock the Stag Lord Fort? Otherwise, I think I wouldn’t have been able to enter the Fort at all.