If this were broken it would almost certainly be something in the firmware that was changed. I vaguely recall, though, that my brother ran into trouble trying to read all 800 bytes from screen memory when he wrote the etch-a-sketch program that I included in NXT Power Programming. It has been so long since I fiddled around with that that I can't remember anymore for sure one way or another. But I am not so sure that it ever worked properly when you try to read all 800 bytes at once.
You could roll back to an older firmware version and/or compiler and see if you can get it to work. That would help me sort out when/where I may have broken something.
Multi-platform LEGO MINDSTORMS programming