Version 6.6.2 of the tools is now available. Please update to the latest version if you are having issues. To leave a comment, see the previous post: DRM Removal Tools for eBooks.
DeDRM_tools_6.6.2.zip can be downloaded from
https://github.com/apprenticeharper/DeDRM_tools/releases/tag/v6.6.2
Release Notes
Another new folder structure, some minor tweaks to the scripts for unusual events, and hopefully a 64-bit Mac OS application that now works from the release download, and doesn’t need an executable bit setting, and doesn’t have an unnecessary splash screen. I can’t do anything about it being unsigned. Apple don’t allow anonymous signing, unsurprisingly. 
Decrypting KFX
Thanks to work by several people, the DeDRM calibre plugin can now decrypt KFX format ebooks from Amazon.
Install the KFX Input plugin to automatically locate all of the files that make up a KFX format book and gather them into a KFX-ZIP. After the DeDRM plugin removes the DRM, then the KFX Input plugin will package it into a single KFX file.
See this post at the MobileRead calibre forum for more information on Dealing with Kindle for PC/Mac 1.19 (and later) and KFX in calibre.