Skyrim letter bug
It's not a big issue.
This letter is a generic dynamically generated quest based on two NPCs. The [...] is suppose to be another NPC's name, but something weird happened with the script and it couldn't pick up the NPC's name.
This isn't going to break any real quests for you, you don't need to worry about it.
The second place where this can show up are inheritance letters when NPCs are killed.