Hi. I’m sure you’ll all thank me for this! The problem is not cookies or cache as some would believe. It is with the .htaccess file in your root directory. WordPress seems to get confused and won’t let you login.
Here’s how to fix the situation:
1. Delete the .htaccess file from your live site
2. Login in to your site and go to ‘Settings’ and make sure you’ve got ‘www.’ infront of both addresses of your blog.
3. Logout properly
4. Re-upload your .htaccess file on your live site in the root
5. Now go to your blog and login!
This worked for me and hopefully for you and will save any heartattack moments after upgrading.
Love to all
Paul