This is the standard logout url generated by WordPress. However, the redirect_to parameter is generated twice. First one is redirecting to the wp-admin/admin.php which is why you see a blank page.
On my test installation it redirects to the home page instead of the admin.php .
We use the wp function to generate the logout link
wp_logout_url , however it seems something is overwriting the logout url.
Please check the third party plugins you may be using, the security plugins etc which might relate to this stuff. Prima facie the issues does not seem to be arising form the theme.