Could still be a DRM removal problem.
Given what you say, the first thing to do when you unzip the stripped .epub is to look at one of the .html files.
It should be easily readable text (in so far as HTML is ever readable).
If not, something has gone wrong in the DRM removal.
For comparison, you can use 7zip or similar on an original DRM .epub, and on a successfully stripped .epub, and look at the .html files.
I have found a small number of library books .epub that opened fine on ADE, but would not open on a Bookeen Opus.
I never found out why, and wasn't that motivated to try to read those particular books.
It may be that there are slightly different ways the DRM is applied, and that some can break both strippers and some ebook readers?
Still may be worth trying another stripper.