Problem with configuration

Hi, my site in loaded in a subdirectory (my_site.it/wordpress/) and this cause a 404 error when I click on “Save” button in plugin configuration page.

I have used this hack

http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory

to show my site at my_site.it

Can you help me to solve the problem?

Thank’s in advance for you reply,
Gennaro

http://wordpress.org/extend/plugins/paypal-express-checkout/