order.php update goes to blank page

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Hi, I've had this problem for a while... when updating an order status in admin, the resulting page is always blank and I have to click back. I've followed a few threads on here for similar problems and using the line echo "this is a test"; I've managed to find a point in the admin/orders.php page where this line no longer shows. This would be around line 38. The echo line shows before:

 

if ( ($check_status['orders_status'] != $status) || tep_not_null($comments)) {

tep_db_query("update " . TABLE_ORDERS . " set orders_status = '" . tep_db_input($status) . "', last_modified = now() where orders_id = '" . (int)$oID . "'");

 

But not after.

Can anyone advise or have a stab at what might be wrong. I've tried replacing the orders.php file(s) with new ones, but I think there's an old add-on which then stops me from viewing the orders list.

 

Thanks

 

Matt