Just upgraded a site to 1.5.4 running on PHP 5.4.39 and had this error whilst testing:

[13-Apr-2015 12:09:16 Australia/Brisbane] PHP Warning: Illegal string offset 'id' in /home/XXXXXXX/public_html/includes/modules/payment/dirbankaus.php on line 50
To fix this open your /includes/modules/payment/dirbankaus.php

Change

PHP Code:
// class methods 
    function update_status() { 
      global $order, $db; 
 
      if ( ($this->enabled == true) && ((int)MODULE_PAYMENT_DIRBANKAUS_ZONE > 0) ) { 
        $check_flag = false; 
to this

PHP Code:
// class methods 
    function update_status() { 
      global $order, $db; 
       
      if ($this->enabled && (int)MODULE_PAYMENT_DIRBANKAUS_ZONE > 0 && isset($order->billing['country']['id'])) { 
        $check_flag = false; 
Note that I am in Australia, if you are using the USA equivalent replace DIRBANKAUS with DIRBANKUSA and similar for other countries.