I have just experienced something similar.
I have 2 DSi's for my daughters and yesterday downloaded Barbie Jet Set and Style. The game worked on one (A)with no problems but not on the other(B).
All the previous loaded games work fine.
Sometimes it says "The save data could not be accessed. Please turn the power off and reinsert the game card." Sometimes it just gives a blank white screen.
I then inserted the other SD card from the working DS (A) and that card worked okay with the aceKard in the DS Lite (B). and vice versa I put the SD card from (B) into (A) and it did not work with the new game therefore pointing to a possible problem with the SD card.
I then reformatted a new 2GB SD card with the Panasonic formatter and copied everything from the old SD card (B) but still the same problem.
I hope this makes sense
Any help appreciated.
thanks