It may no loner be of interest to you but, you could try using ebook DRM removal tools - they add into calibre and may solve the issue
https://apprenticealf.wordpress.com/2014/12/21/tools-v6-1-0-posted/
https://apprenticealf.wordpress.com/2012/09/10/drm-removal-tools-for-ebooks/
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Posts made by mmi
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RE: Converting books - questions, help, advice