Please help! Problems with my post preview

I can publish a post, but I can’t preview while I am writing. What I see when I use the preview button is my blog, and in the place of the new post, the message “Not Found
Sorry, but you are looking for something that isn’t here.”

I think that must be a problem with the .htaccess file.
I have the wordpress installation in:
http://solaiz.com/aldares.com/wordpress
When in options of my blog, I set :
WordPress address(URL):http://solaiz.com/aldares.com/Wordpress
Blog address (URL):Blog address (URL):http://solaiz.com/aldares.com/Wordpress
the preview works fine.
But when I change the Blog address (URL)to http://www.aldares.com (that is a domain that is pointing to http://solaiz.com/aldares.com/Wordpress) then the preview (while I am writing) show the error message: Not Found
Sorry, but you are looking for something that isn’t here.
What I have in my .htaccess file is:

RewriteEngine on
Options +FollowSymlinks
RewriteBase /
RewriteCond %{HTTP_HOST} /aldares.com/WordPress/
RewriteCond %{REQUEST_URI} !/aldares.com/WordPress/
RewriteRule ^(.*)$ aldares.com/WordPress/$1 [L]

Someone can see something wrong?
Thank you in advance!