Strange problem with updates (Wp Admin)

[Igal]

Hello everyone,

I'm trying now to replace my 468x40 post banner (adsense) with an Amazon 468x60 banner.
I followed the instructions on this tutorial:
http://studiopress.com/support/viewtopi ... =18&t=4378 and everything was perfect until I tried to "Save Changes" in the Widgets.

When I click on "Save Changes" I'm forwarded to a 404 page - if I go back and try to save changes without Amazon's html code, everything is fein.

I also tried today to update my index.php file and I was also forwarded to a 404 page (eventually I modified it with the Winscp)

Any idea what might cause it?

Thank you

Here's the site: http://www.lcd42.co.uk