What is apparently the first "game-breaking bug" in Fallout 4 has been reported on the Bethesda forums. According to user JoeBBBB and several other posters, traveling to Monsignor Plaza can cause a crash to desktop, with no error message reported.
The initial complaint, which actually goes into quite a bit of depth about how it all works, says the problem appears to come up once players have passed a certain point in the game (the post goes to lengths to avoid spoilers, as do I), or after they've unlocked/passed a certain location. The problem is compounded by the fact that Monsignor Plaza is a location in a settlement quest, and once that quest is accepted, you're stuck with it.
Starting a new game doesn't seem to help, nor does reinstalling and then loading up old saves, and it doesn't matter which direction you enter the area from or how you get there. Going back to a pre-crash save won't make the bug go away either, although it sounds like it will at least delay the problem: "If you go back to an earlier save with the crash, you'll probably find you don't crash there so it happens at some point later," the message says.
It's not unusual for a Bethesda game to be buggy at launch—it would be unusual if it wasn't, I think we'd all agree—but so far the problems have been limited to relatively minor glitches: Irritating (and sometimes amusing), but not fatal. This bug, however, is clearly more serious. I've asked Bethesda if it's aware of the problem and, if so, if there's any kind of time frame for a fix, and I'll let you know what I hear back.
