Can't retrieve the data from database

Question

JSON-String is empty but why? The same code works fine if I change the table. There are 20 entries and the Code works today afternoon but for some reason I can't get the entries anymore :(

    <?php

    // open connection to mysql db
    include("connect.php");

    // fetch table rows from mysql db
    $sql = "SELECT * FROM Videoquestions";
    $result = mysqli_query($conn, $sql);

    // create an array
    $var = array();
    while($obj = mysqli_fetch_object($result)) {
    $var[] = $obj;
    }

    // convert array to JSON
    echo '{"Videofrage":'.json_encode($var, JSON_UNESCAPED_SLASHES).'}';

    // Close the database connection
    $conn->close(); 

    ?>

Need to encode to utf8 ... anybody knows how to? German Umlaute like äöü are blocked and with this line of code it will be solved:

mysqli_set_charset($conn, 'utf8');