I had this same problem and what worked for me was to download an old version of Kindle for PC, regardless of the age of the books. I think version 1.17 works. Have copies download to it and then try the Calibre conversion again.

This is why pirating is alive and popular, because it makes people's lives convenient, while the real sellers do many things to make people's lives difficult. You as a buyer are expected to bend and give in to their way of usage and their restrictions.