Repair 406 with a textarea inside a form

Question

I have a form with a textarea and an input, but when I submit it, I got error 406 on my server,

I don't have this error on my localhost,

Here's my code:

if( isset($_GET["action"]) && $_GET["action"]=="send_email" && isset($_GET["message"]) )
{
    $message = $_GET["message"];
    $t = mailContact( $db, $message, ACTION_USER );

    redirect($t?"./?message=email_sent":"./?message=email_notSent");
    exit();
} 

and

echo "<form method='get'>" ;
echo "<input type='hidden' name='action' value='send_email'>" ;
echo "Message:" ;
echo "<br>" ;
echo "<textarea name='message' id='message' style='width:100%;height:200px;'>" ;

echo "Hello,\n" ;
echo "I want to tell you :\n" ;
echo "\n" ;
echo $_SESSION["name"]."\n" ;

echo "</textarea>" ;
echo "<input id='submit' type='submit' value='Send' style='width:200px;'>" ;
echo "</form>" ;