The only thing of note is DeDRM attempting to decrypt the non-DRM'd file. Disabling DeDRM doesn't resolve the issue and in that case I get nothing in debug log past all the Calibre startup messages.
Code:
DeDRM v10.0.9: Trying to decrypt george-grey_polynesian-mythology.epub
DeDRM v10.0.9: Verifying zip archive integrity
DeDRM v10.0.9: Post-processing took 0.0 seconds
DeDRM v10.0.9: Finished after 1.9 seconds