SOLVED:
Don't follow other comments telling you to delete specific files such as "x64a.rpf" and don't redownload the entire game again.
The Rockstar downloader doesn't run checksums over it's downloaded files (god know's why?!?!). This is has been discussed here: http://www.reddit.com/r/GrandTheftAutoV_PC/comments/328ljj/found_a_fix_for_the_social_club_download_reset/