So okay, I'm very sorry for the confusion, I have difficulties to describe the issue as it depends on several factors, and it is slippery like fish; so I hope you bear with me. I made a protocol of the behaviour of the forementioned structure:
35: FE-user area, NOT recursive
`- 36: BE-user section
`- 38: regular page
These pages behave like the following (with a debug-echo in checkRootlineForIncludeSection, which is the only function I found which actually handles doktype==6):
--------------------------------------------------------------------------------
--- Case one, accessing a page which is "BE-User Section"
Pages-cache deleted, admin-user logged in, load page:
- checkRootlineForIncludeSection(): this->beUserLogin is 1
- Content visible
Admin-user logged out, reload same page:
- checkRootlineForIncludeSection(): this->beUserLogin is 0
- Content visible!!!
Other BE-user logged in, reload same page:
- checkRootlineForIncludeSection(): this->beUserLogin is 1
- checkRootlineForIncludeSection(): this->beUserLogin is 0
- Content visible
--------------------------------------------------------------------------------
--- Case two, accessing a page which is child of a "BE-User Section", but
--- itself is a regular page
Pages-cache deleted, admin-user logged in, load page:
- checkRootlineForIncludeSection(): this->beUserLogin is 1
- Content visible
Admin-user logged out, reload same page:
- checkRootlineForIncludeSection(): this->beUserLogin is 0
- Content visible!!!
Other BE-user logged in, reload same page:
- checkRootlineForIncludeSection(): this->beUserLogin is 1
- checkRootlineForIncludeSection(): this->beUserLogin is 0
- Content visible
--------------------------------------------------------------------------------
Okay, so I believe the "BE-user section" suffers from caching-artefacts, I compared the code with down to 4.0.4 and it didn't change. So my guess is that page-handling & caching changed, ignoring the case of doktype==6.
All the problems I mentioned here occured in relation to a "BE-user section", some don't occur anymore, possibly because of cache-side effects too.
So much about that. Sorry it's just super-weird behaviour, I can give access to a Typo3-SVN installation which behaves like that, if somebody wants to dig into the real case.