wp_login action and wp_mail

So far, I’ve been workung with WP3.5.1 and decided now to update to WP3.9.2. Yet, I am facing problems with login/logout actions/hooks.. I am using the actions as recommended like so:

add_action('wp_login', 'PN_on',10,1);

function PN_on($user){
	global $wpdb;
	$GLOBALS['PM'] = $user;
	$us = get_user_by('login', $user);
	add_user_meta($us->ID,'login',$user );
	wp_mail('mail@to.me', 'PM_signon', dbg_r($us->user_login));
	if ( in_array('administrator',$us->roles ) ) {
//		wp_mail('mail@to.me', 'PM_signon_Admin', dbg_r($us->user_login));
		}
}

When I try to login, the process is not completed and -in debug mode- I get a message output like ‘cookies are de-activated due to unexpected output’ and further on the infamous ‘cannot complete header’-error and finally login hangs up.
If I comment out the wp_mail lines, everything works perfect.
It worked until WP3.5.1 but obviously no longer now.
How can I send an email during login process on certain conditions in WP3.9.2?
Should I use ob_start here instead?