Hi. There seem to be a new version of dbtoepub available from Sourceforge. Please update the version in Debian to the latest version. The list of commits in the upstream subversion repository can be seen on <URL: http://sourceforge.net/p/docbook/code/commit_browser >. I hope a new version might fix some of the problems I see with epubcheck in my book.
I think this project is now at github: https://github.com/docbook/xslTNG seems Sourceforge has been abandoned for over 17 years.