I had this issue too. So eventually after trying on a new account and everything else I succumbed to deleting GT5 DAta in the 'Game Data Utility'. Naturally, backing up my game progress in the 'Save Data utility' first'.
I think the issue was that I hadn't received a prompt to download a new 'System update' before I downloaded the game's patch.
So
1. Download the latest System update
2. Save your GT5 progress (back it up to a USB stick) from the 'Save Data utility', in case needed.
3. Delete your massive Game data file.
4. Restart the game and re-perform the huge 'bulk download' 40 mins approx
5. Start up your game.
It works again.
Pain in the butt, yes, better than losing everything though.
Good luck.