Dawnguard Quest "Failed to Look up"

[PlanetSlayer]

Hi, I'm PlanetSlayer

I'm having an issue with Skyrim where; when I speak to a guard and he/she provides the generic 'I might join the Dawnguard' line, which would normally end with me getting the quest labelled Dawnguard, instead the quest is re-labelled "Garen Marethi" which, is a character from Dawnguard. When I go into the quest journal, instead of the quest goal "Join the Dawnguard", the words "Feran Sadri" are there. Above that, where the quest description would be, is where the words "Failed to look up" appear. A screenshot is provided below.

My game is completely mod free, besides the Unofficial Patches for Skyrim and all its gameplay DLC, which I've also got activated, because I've been working on a clean savegame resource lately. This bug has been an issue for at least 3 weeks, both with and without a modded Skyrim. My Skyrim is version 1.9.32. It doesn't matter whether my DLC masters and Update master are cleaned with TES5Edit or not. I'm playing with a legit, store bought, english version of Skyrim.

PC specs
Operating System: Windows 7 Home Premium 64-bit Service Pack 1
BIOS: Award Modular BIOS v6.00PG
Processor: AMD Phenom II X6 1055T Processor (6 CPUs), ~2.8GHz
Memory: 16GB RAM
DirectX Version: DirectX 11
Graphics Card: NVIDIA GeForce GTX 650 (2GB VRam)
Sound: Nvidia High Definition Audio

I've researched this a bit and the only good advice I could find was this:
http://forums.nexusmods.com/index.php?/topic/635807-lookup-failed-bug/

However, the solution suggested in the link hasn't worked for me, I've tried validating steam files, like it says to, atleast 10 times , since I originally got this issue, however, it was mostly for other reasons then. I have done it with the intention of fixing this. I've done a semi re-install of Skyrim atleast twice. Which means, I copied and replaced the Skyrim folders from a pristine, clean Skyrim zip archive.

Here is a picture of the bug:
http://cloud-2.steampowered.com/ugc/866094026162417481/B4984EAC8D336466024ACA82CE4D3FB1FAFAEF49
It's a bit too big to put in the text field.

Please, note; I haven't started any DLC quest besides this and Hearthfire before, so avoiding spoilers would be appreciated.

Edited May 19, 2013 by PlanetSlayer