Just to add to this, I'm getting the same thing. Fast travelling to and/or walking into Sanctuary, I get a CTD.
1. I've tried without mods enabled, same crashes.
2. Walking as close to, fast travelling away, fast travelling back - same crashes.
3. I can fast travel to Hangman's Alley, which has Rise of the Commonwealth city manager set up in the same manner as Sanctuary, and that doesn't crash.
- 3b. I can walk to Red Rocket, but can't fast travel there. I haven't added anything to this settlement at all, so not sure if that's relevant or just that's it's close to Sanctuary. Indeed, it's almost before I walk out of Red Rocket toward Sanctuary that the crash occurs.
I'll upload the save file, if required (with this in mind, do you want mods disabled for any uploaded save files - bar official and SS esps/esms?)
I've tried the Fallrim tools, and am on 3.0.3b - the active scripts section has no SimSettlements scripts there.