Error 400 Bad request

[CommanderRedYT]

On every file upload I get an error 400. Tried to put some error_log() into the code, but nothing showed up. The error message comes from this:

if ($file->validateChunk()) {
     $file->saveChunk();
} else {
    // error, invalid chunk upload request, retry
    header("HTTP/1.1 400 Bad Request");
    return ;
}

But I don't know why there is an error...
(Using the example-code from the node-js sample (index.html))