Since I updated the WordPress from a friend to 3.5.1 (configured as Multiblog) a PHP Error-Message appears while "Attachments" is activated on backend and frontend of the blogs:
Warning: Invalid argument supplied for foreach() in /www/htdocs/multiblogs/wp-includes/query.php on line 1702
There are some other Error Messages from other wordpress functions and "header already sends" too, but all others refering query.php on line 1702 as source of the evil.
If I turn Attachments off it works, if I downgrade wordpress (thanks backup) it works.
I will test the next days if this appears on my own WordPress-Blog too if I update to the latest WP Version.