Hi,
It actually happened again this evening on a new save file.
When you click on 'store' or an unlock prompt appears ie dodgy lasagne, you go to the store screen, when you click on the cost button 99p etc, a purple rectangle appears in the centre of the screen immediately, stating something to the tune of unable to make purchase error:1. At this point you have to press x button to exit, this happens for all the various unlocks.
This was happening on my xbox since Christmas day so just assumed it was a bug that would be sorted in time, i noticed that you had been working on u