If your old backups still work, it means that you still have LT on your drive. Most likely your backups are failing new AP2.5 checks. Re-rip/re-burn your backups with the new dae.bin.
Edit: It looks like dae.bin is being updated on the fly now and is now console specific.http://www.xboxhacker.org/index.php?topic=17343.0
It looks like the only way to play safe backups at the moment is to
A. Disconnect from live.
B. Grab your personal dae.bin from your nand.
C. Re-rip your original with 0800 drive and your dae.bin.
D. Reburn, play.
E. Stay off live altogether.