L-1410: Ashes to Ashes, Rust to Rust - courier interruption bug
Seems to be caused if the courier's dialogue/greet is interrupted, e.g by initiating dialogue with another NPC. The letter is added to the player's inventory, but the quest doesn't appear in the pip-boy, so there is no indication of what to do or where to go. The courier will give his dialogue when interacted with, but it doesn't make the quest trigger. The quest is actively running - it just appears "invisible".