I'v tried following the instructions for
- .htaccess (helps to get the editor for admin only but not for teachers)
- file permissions
- data path outside and inside documentroot
- data acces with/without hash parameter
- check for data compression activation at provider
- comparing mdl_config tables of working instances (on the same virtual server!)
- comparing activated modules, php.ini ...
Nothing more to do than a new-install or try an upgrade first.