logout is triggered at the entrance to the wp-admin.

No idea, but in my case, in such a situation, I’d use my FTP client to first deactivate all plugins ( http://codex.wordpress.org/FAQ_Troubleshooting#How_to_deactivate_all_plugins_when_not_able_to_access_the_administrative_menus.3F )

If it doesn’t help, you can switch back to the default wordpress theme (FTP, wp-content/themes, rename your current theme to something else, like adding an underscore, wordpress will switch back to the default theme, try F5 a few times).

If nothing works, consider a very dirty reinstalling of wordpress ? FTP, delete /wp-admin and /wp-includes, reupload them by FTP from a fresh wordpress.zip downloaded off wordpress.org…