Well done for calibre software which I really enjoy using.
I downloaded calibre version 5.10.0 on iMac (Retina 5K, 27 inches, late 2015). Since I upgraded to macOS 11.0 Big Sur, I can no longer read books with DRM that I add to the library. I wanted to revert to calibre 4.23, but it doesn't work either. I have an error message which refers me to the page
https://manual.calibre-ebook.com/drm.html. Have you planned to modify or extend the program?