Error 404 - Not found

I have problems using the fckeditor on one of my pages. I instoled the editor using the integrated mambots install routines of Joomla (latest stable)
The integrated Tiny MCE works fine but I am missing some functionalties. When I open now a content which is existing i get a "Error 404 - Not found"
Message in the screen where the Editor should load.
Doing the some on another Webspace is working fine. Both are the same fckeditor versions and Joomla versions.

The different is the following:
working version:
http://www.domainname.com/administrator ... nu=1&id=16

not working version:
http://www.domainname/Sub/administrator ... nu=1&id=14

You see the uppercase letter of the "Sub" folder! this was a mistake while installing everything but to change this would men a complet now installation of Joomla which I don`t like. "EDIT" changed the folder to lowercase and changed the configuration.php. Now can load the page in lowercase but still get the 404 error!

But on the other hand all other plugins are working fine.
Any suggestions?

Andreas