klaudius

The bug was caused by a change about Log out mechanism with was reffered in

http://codex.wordpress.org/Migrating_Plugins_and_Themes_to_2.7

which said:

Logout from Site

The Template Tag wp_logout_url was added in Version 2.7 to provide a nonced URL for logout purposes. If a theme uses a construct such as /wp-login.php?action=logout to offer the user a logout link that code should be updated to use the wp_logout_url. A good example of the usage of wp_logout_url can be seen in the WordPress Default and Classic themes comments.php and comments-popup.php Templates.

Users will experience a You are attempting to log out of example.com. Please try again. message if the theme does not use wp_logout_url for the logout URL.

So just find one line in your current theme files

<a href="<?php echo get_option('siteurl'); ?>/wp-login.php?action=logout" title="Log out of this account">Log out</a>

and replaced these codes with

<a href="<?php echo wp_logout_url(get_permalink()); ?>" title="Log out of this account">Log out</a>

You can try log out again, and no same bug occurs again.