I havehad the same problem with my kobo glo HD. this is with that epub https://docs.spring.io/spring-boot/docs/2.0.0.M5/reference/epub/spring-boot-reference.epub It's It's drm free. I have tried to delete and re upload the file and it's still considered locked with drm. Then I have tried to open it with fbreader(on my desktop) it works, no drm as expected. then I have tried to download and open file from kobo web browser as describe in your advice and still considered as a drm file.
Finally I tookbut not with that one from redhat for testing and it's readablemany other epubs :
Why spring boot epub is considered with drm... :(I have tried to download from kobo web browser, upload from my computer many times and it was not working so I did a check on
next step is tryinghttp://validator.idpf.org/
and it tells me content.opf were missing. I checked it by my self and the path was incomplete in the META-INF/container.xml file. So I wrote the full path to search diff or remakecontent.opf did a new check then upload the epub but if you have any solution meanwhile it's welcomefile on my kobo again and it works.