> For me, a theme is just a presentation layer, nothing to do with the precess behind.
That is basically correct. The other themes don't do anything different than basic theme. The only main difference is that Basic does not support the optional Zenpage CMS plugin.
This is confusing but I still cannot reproduce as it is all working in all official themes as intended for me.
So did you try modrewite instad? You are still using the nonmodrewrite urls. Maybe that gives us a clue.