I've been having the same problem for a few months now, so I decided I'd had enough and just called Sony. After dealing with 3 people who had no idea what I was talking about, I finally just went to a supervisor and talked with them for a good half hour of listing things to try. Among them were delete the 'Game Data Utility' for the game and reinstall it, explicitly sign out and back in again (even if you turn off your system, supposedly Sony's servers don't actually kill your connection to the network for some reason, according to the supervisor I was talking to. Surprising, I know..), and deactivating/reactivating the system for game and video content.
I tried deleting the Game Data Utility and reinstalling it, and after suffering through the installation and updating process, I was able to make a purchase and begin a download. Once. That's right. It worked once. My three episodes of the Super GT that I tested the theory with are happily downloading to My List as I type this, and I'm now receiving "An unexpected error has occurred" again.
So, lesson here: write down everything you want to download, delete the Game Data Utility (NOT the "Saved Data Utility" for the love of god) for GT5P, reinstall it, update it, put everything back in your GT5P cart (as it will be cleared out), and download/purchase EVERYTHING at once.
I forgot to mention that by deleting the Game Data Utility for this game, it will not only delete your cart, but it will also delete all your options settings, so if you do a lot of customizing of your options, you'll have to do all that over again. I figured that'd be good to know.
HTH
