I've searched the forum, and seems that nobody is having a problem but me. I installed the newest version of Calibre portable (my first upgrade since v 8.38) with the intention of using the Subset Fonts feature.
My first attempt, converting epub to epub, the conversion completed, but deleted all four embedded fonts and @font-face statements, but retained the references in the CSS styles.
I reimported the book to Calibre. The next attempt, the conversion aborted. The traceback said it subsetted the fonts ok, but aborted upon deleting the FOURTH AND FINAL original font, with an error code of "1".
I thought perhaps that font was corrupt, so deleted it in Sigil, deleted the book from Calibre and re-imported. This time the conversion aborted at the point of deleting the THIRD AND FINAL font, with an error code of "1".
Before I run myself crazier deleting fonts and running the conversion multiple times, I am begging for help.
I'm attaching the traceback message.
I hope somebody has good advice for me, it will win you gold stars in heaven!