[Ken]

I experienced the same thing, on the same day, on two separate computers, and the second computer did not receive a Kindle update, it's still v1.8.3.

"Error: No key found. Most likely the correct PID has not been given."

And I get the same result when using a trial version of 'Kindle DRM Removal', so it doesn't seem to be a Calibre problem. If you find a solution, I'd sure like to hear about it.

Ken