I had to open my LX20 last night to replace both the 9v NiCd which
wasn't holding a charge with a 9v NiMh, and the small watch battery
which was dead. I reassembled it, connected it to the charger, and hit
the ON button.

It starts beeping and displays the message "Secret Key Not Valid!"

I followed the Memory Init routine from the manual, and put in a new
password in the Setup menu, but after I turn it off/on, the error
returns.

What do I need to do now?

Thanks,
Ken