general.php is trying to use the header('Location: ' . $url); to perform a re-direct. This will not work is you have output any text to the display either through HTML code or an echo function. It is telling you that the output to the screen is at line 551.
You have debugging turned on, and I have found that debugging causes virtually everything to fail, because of general.php trying to use the header function. If you really need to do the debugging you could change the header function to an META HTTP-EQUIV call.
<meta http-equiv="Refresh" content="0;<?=$url?>">
Not tested.
New 
