Have you done anything to the box after installing the custom firmware? I suspect you're running an older rc.local that doesn't properly modify the passwd file.
To temporarily get into the box, make sure you flash the xxx_02 or later nas file and press and hold the copy button when the ready light blinks (three times) until it remains on steady. This will bypass any customizations and force the firmware default startup scripts which should work for you.
- K.C.