By Post Field 'query args' with author

Do you mean query posts created by current user? Please clarify which user you want to set as author.

I think the problem is calling get_current_user_id too early. This docs page has an interesting comment:

Before the plugins_loaded action it returns 0.

So you might want to make some change to:

add_action( 'init', function() {
     add_filter( 'rwmb_meta_boxes', 'your_register_function' );
} );

Assume you have this code to register your meta box:

add_filter( 'rwmb_meta_boxes', 'prefix_register_meta_boxes' );
function prefix_register_meta_boxes( $meta_boxes ) {
    $meta_boxes[] = array(
        // Your meta box code goes here
    );
}

Then change it to:

add_action( 'init', function() {
     add_filter( 'rwmb_meta_boxes', 'prefix_register_meta_boxes' );
} );
function prefix_register_meta_boxes( $meta_boxes ) {
    $meta_boxes[] = array(
        // Your meta box code goes here
    );
}

Hey I got the same problem but I registered the Custom Field that contains the Post Field that I want to quary arg for the author = user with your MetaBox AIO Plugin and the GUI

I dont know where i have to put those i register the metabox with your plugin

Assume you have this code to register your meta box:

add_filter( 'rwmb_meta_boxes', 'prefix_register_meta_boxes' );
function prefix_register_meta_boxes( $meta_boxes ) {
$meta_boxes[] = array(
// Your meta box code goes here
);
}

Then change it to:

add_action( 'init', function() {
add_filter( 'rwmb_meta_boxes', 'prefix_register_meta_boxes' );
} );
function prefix_register_meta_boxes( $meta_boxes ) {
$meta_boxes[] = array(
// Your meta box code goes here
);
}