Are you using the same hardware for both the RD2BN and the 668? If so then it sounds like an electrical problem. Check the voltages on VCC, GND, /PSEN (during reset), RST (during and after reset), P2.6 and P2.7 and make sure they are all what they should be, and glitch and ripple free.
If you are not using the same hardware then between the two successful programs of the RD2BN, did you execute the downloaded code?