Submissions of bulk modifications to change user roles from the User Management page always results in the creation of a bad wp_redirect URL.
The redirect URL *should* be:
/wp-admin/users.php?update=promote but the actual URL created is: /wp-admin/users_php&update=promote
The function responsible for mangling the URL is add_query_arg(). The function is called from within wp-admin/users.php (around line 147). The $redirect URL is "users.php" before the call and becomes transformed to "users_php" after the call to add_query_arg(). The '?' is stripped or replaced with '&'.
I've temporarily written a 'filter' for wp_redirect that replaces '_php&' with '.php?' - someone should fix this.