ISSUE REPLAY: Content if NOT FOUND not displayed properly on 404 page

I raise the issue once again as there were no working solutions yet. The previous thread is 8436 (http://forum.bytesforall.com/showthread.php?t=8436)

Wordpress 3.0.1 Atahualpa 3.5.3

Permalink structure defined as: /teksty/%postname%/

Issue: When calling nonexisting page or post on my site I got the page with the list of excerpts instead of the content defined for 404 page. The page tittle is as for 404 page, however.
Example: http://piotrcelinski.info/non-existing

The proper content for 404 page is displayed when calling the sample page http://piotrcelinski.info/teksty/non-existing

You may compare the results for the above links

Most likely this is not caused by any plugin I use neither by the .htaccess file - I tried to disable plugin by plugin as well as use the default .htaccess only.

As you see the permalink structure was changed. I tried to come back to default structure and then to the defined - as juggledad advised, but there were no results.

Any solution?

Regards
Piotr