Coby may have recently updated the update to cause this. I think you'd be better off rooting and installing clockwork with the firmware that came with your device then backing up and either installing an older 7015a update or one of the custom ROMS. Would hate to see you install the update from Coby site then be locked out of applying unsigned zips.
Can anyone confirm this is a problem? I know I'm not willing to try it then be locked out.