Error when I update Plugin, Change Admin Password…

I keep getting this message when I try updating plugin, changing Admin password and a few other things, “No Results Found
The page you requested could not be found. Try refining your search, or use the navigation above to locate the post.”

I disabled all the plugins and tried changing the Admin’s password, the message still comes up. I already posted on the forums and was advised to do this.

– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).

– switching to the Twenty Eleven theme to rule out any theme-specific problems.

– re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.

The only thing I didn’t try is,

– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.

I renamed the plugin folder and the problem still occurred.

Anyone have any ideas what I can do to fix the problem? Thanks!