After upgrading Wordpress to 2.9.2, I wasn’t able to login at the admin page of this blog.
By disabling all plugins I was able to login again. After that , I have enabled every single plugin, one by one, until a found out that qTranslate was the cause of this failure. (Google for “Disabling Wordpress plugins” if you don’t know how to do that).
A few hours after Googling for a fix, I couldn’t find any useful information. However, this error happened with me in the past and it as due to the desynchronization of the “AUTH_KEY” on wp-config.php file and the “wp_default_secret_key” in my language file (wp-content/languages/en_US.php).
I went to the en_US.php file and it was ok, the keys were equals. So I deleted this file, just to see what would happens. Voilá! It was working again.
So, the fix is as simple as:
rm /<installation folder>/wp-content/languages/en_US.php
March 5th, 2010 at %I:%M %p
I got this issue after upgrade and this helped me, thank you!