So I went ahead and uninstalled my copy of the game, downloaded the client from the website and installed it though that and redownloading all of the required files. I installed it on my Main Drive(C:) this time as opposed to having installed it on my Secondary Drive(D:), and now I could fight Titan NM and progress in the storyline. I haven't tried The Sunken Temple yet, but at least now I'm happy that I can progress again, over 10 levels over the main target level for the storyline :) Makes things a lot easier and quicker though.

Thank you for your time in investigating this! I'll be back if there's anything else regarding this issue.