So, if you are like me and usually access your WordPress admin Dashboard by going to /wp-admin/, then you probably also have troubles with login not working. Mine stopped working with the latest version install (3.2.1).
What happens is /wp-admin/ redirects to wp-login.php with a bunch of parameters, the last one being “reauth=1”. If you look closely, you’ll notice that there’s also a redirect_to parameter, which should look like yoursite.com/wp-admin/. If you have your WordPress installed in a subdirectory, and not in the webroot, this redirect_to parameter might be set incorrectly. Thus – continuous loop from wp-login.php to /wp-admin/, and /wp-admin/ can not be found.
One way to get rid of this is to login from wp-login.php page directly. The other one is to tweak some WP settings. I’ll update the post once I know exactly what needs to be updated and where.
A helpful WP post on how to troubleshoot login issues: http://codex.wordpress.org/Login_Trouble
Tanya Nam 
Heya … I had the same issue …. And the problem was that cookies were set to be blocked in my router after I did a router reset. Took me ages to figure out but solved it in the end…
I had this problem as well, when migrating a blog from one hosting service to another. Had to go into the database via phpmyadmin and change the blog’s URL from the old site’s URL to the new URL.
Same problem and applied change in siteurl as suggested (wp_options table).
Working fine now, Thanks!
Great – glad it was helpful!
Superb post but I was wanting to know in case you could write a litte a lot more on this topic? I’d be extremely thankful should you could elaborate a bit bit far more. Thanks!
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I had the same problem with my gaming website. After some digging, I’ve found out what it was. First I checked error log and there it was PHP Warning: mysql_real_escape_string() expects parameter 2 to be resource, in file wp-db.php
The problem had been caused by depricated function mysql_real_escape_string after hosting provider updated PHP. So I simply changed that function with esc_sql in wp-db.php and it worked.
It is very likely that your server’s PHP was updated and that some functions became depricated.
Hope that helps.
I think this rule should work in the .htaccess file
RewriteRule ^wp-admin/(.*)$ /wp-login.php?$1 [R=301,NC,L]