I get an error when logging

The error that it gives me is when trying to access apanel.php sometimes it accesses and sometimes it does not, I think it's thing of the sessions but I'm not sure.

PHP:

<?php
session_start();
        include("cabecera.php");
        include("conexion.php");

    if (isset($_REQUEST['Enviar'])) {
    $usuario = $_REQUEST['ausuario'];
    $password = $_REQUEST['apass'];

    $sql = $conexion->query("SELECT * FROM usuarios,apass WHERE ausuario='".$usuario."' and apass='".$password."'");
    while ($login = $sql->fetch_assoc()) {
        $usuarioDB = $login['ausuario'];
        $passwordDB = $login['apass'];
    }
    if ($usuario == isset($usuarioDB) && password_verify($password, $passwordDB)) {
        $_SESSION['loggedin'] = "loggedin";
        $_SESSION['ausuario'] = $usuarioDB;
        $_SESSION['apass'] = $passwordDB;
        header("Location: apanel.php");
    } elseif ($usuario !== isset($usuarioDB)) {
        echo "<div class='error'><span>El Nombre de Usuario que has Introducido es Incorrecto</span></div>";
    } elseif (password_verify($password, $passwordDB) === FALSE) {
        echo "<div class='error'><span>La Contraseña que has Introducido es Incorrecta</span></div>";
    }
}
?>

HTML:

<form id="login-form" class="text-left" method="post">

<br/>

<tr>
                <td><span class="fa-user"></span><input name="ausuario" placeholder="usuario" type="text" onfocus="clearText(this)" onblur="learText(this)" class="textfield"/><br/><br/></td></tr>

<tr>
<td><span class="fa-lock"></span><input name="apass" placeholder="contraseña" type="password" onfocus="clearText(this)" onblur="clearText(this)" class="textfield"/><br/><br/></td></tr>

        <input type="checkbox" id="lg_remember" name="lg_remember"><label for="lg_remember">recordar</label><br/>
                                        <tr><td><input id="submit" type="submit" name="submit" value="Enviar">