Problem with join query PHP

Good,

And the following I have a query JOIN that makes me the query to 3 tables, that searches me in the bank for all the notifications of the followers of the currently logged in user. The problem is that I was now trying to fetch information from the user who sent the notification, to show in the notification but the query stops returning me data. From what you can see it returns 500 error server.

Code

header('Cache-Control: no-cache, must-revalidate'); 
//Alteramos o cabeçalho para que o retorno seja do tipo JSON 
header('Content-Type: application/json; charset=utf-8');

session_start();

require_once("../funcoes/funcoes.php");

$user_actual = $_REQUEST['user_logado'];


$result_notificacoes = $conexao->prepare("SELECT * FROM notificacoes as noti 
                                    LEFT JOIN notificacoes_visualizacao as noti_vi ON noti_vi.id_notificacao = noti.id 
                                    INNER JOIN users_social as user ON user.id = noti.user_id
                                    WHERE noti.user_id IN (SELECT seg.followed FROM seguidores as seg WHERE seg.follower = :user_actual) 
                                    AND noti_vi.id_usuario_que_visualizou is NULL 
                                    OR noti.user_destino_notificacao = :user_atual2");                                  

$result_notificacoes->bindValue(':user_actual', $user_actual, PDO::PARAM_INT);
$result_notificacoes->bindValue(':user_atual2', $user_actual, PDO::PARAM_INT);
$result_notificacoes->execute();

$return = array();

$notificacoes = $result_notificacoes->fetchAll(PDO::FETCH_ASSOC);

foreach ($notificacoes as $row_notificacoes) {

    $sql = $conexao->prepare("INSERT INTO notificacoes_visualizacao (id_notificacao, id_usuario_que_visualizou) VALUES (:id_notificacao, :id_usuario_que_visualizou)");
    $sql->bindValue(':id_notificacao', $row_notificacoes['id'], PDO::PARAM_INT);
    $sql->bindValue(':id_usuario_que_visualizou', $user_actual, PDO::PARAM_INT); 
    $sql->execute();

$return[] = '
    <table border="0" class="barra_notificacoes" cellpadding="0" cellspacing="0">
        <tr>
            <td valign="top" width="10%">
                <div style="margin-left: 4px; margin-top: 5px;"><img width="50" height="50" src="images/avatar6.jpg"></div>
            </td>
            <td valign="top" width="50%">
                <div style="margin-left: 5px; margin-top: 2px;"><a href="users/<?= $row_user_anex->slug; ?>">César Sousa</a></div>
                <div style="margin-left: 5px;" class="comentario_user">Comentou a tua opinião!</div>
                <div style="margin-left: 5px;" class="tempo_do_post">1 segundo atrás</div> 
            </td>
        </tr>
    </table>
    ';                  
}
echo json_encode($return); 

The problem is when I add this line, the query stops working

 INNER JOIN users_social as user ON user.id = noti.user_id

Error log

 sabeonde/public_html/ajax/mostra_notificacoes.php:22
 Stack trace:
 #0 /home/sabeonde/public_html/ajax/mostra_notificacoes.php(22): PDOStatement->execute()
 #1 {main}
 thrown in /home/sabeonde/public_html/ajax/mostra_notificacoes.php on line 22

Try Error

 constraint fails ('sabeonde_sabeonde'.'notificacoes_visualizacao', CONSTRAINT     'fk_notificacao_visualizacao_notificacao' FOREIGN KEY ('id_notificacao') REFERENCES  'notificacoes' ('id'))' in /home/sabeonde/public_html/ajax/mostra_notificacoes.php:41
 Stack trace:
 #0 /home/sabeonde/public_html/ajax/mostra_notificacoes.php(41): PDOStatement->execute()
 #1 {main}