Error 404 whilw I upload images

[D.O.]

Sorry, it was a mistake: it happens only when I try to UPLOAD files and images and at the moment my webserver has even changed error message (!): HTTP Error (406)

Usually I use IIS webserver but this one is a Apache.
I don't understand why it doesn't work :-(
Everything is "green" in the "Website Health Check" section.
Anywya, I hope these info can be useful:

1) GetSimple Version: 3.2.3 Latest version installed
2) PHP Version: 5.3.3-7+squeeze15
3) Apache web server: Apache/2.2.16 (Debian) mod_fastcgi/2.4.6 mod_ssl/2.2.16 OpenSSL/0.9.8o mod_fcgid/2.3.6 PHP/4.4.4-8+etch6