Mac store app vs. the app downloaded directly from Amazon? I seem to recall people having problems with that in the past.
At any rate, if you look at the first log in that
issue, you can see how the key isn't found. It appears as if they don't have Kindle for Mac installed at all, but they did. I've seen the same thing on Windows when people have version 1.25 of Kindle for PC.
(Yeah, can't see stand-alone DeDRM vs. plugin being the difference either. Same exact scripts, just running on different Python.)