PayPal Plus: Order status set to "processing" and not "on-hold"

Yes!

[payment_status] => Completed

What do you mean by adding a mail in which function?
What code do I have to put where? (sorry for my inexperience)

The chunk of code that sends the email in the function I sent you as a catch-all…

$to = '[email protected]';
    $subject = 'Email from PayPal IPN';
    $message = 'The IPN was hit';
    $message .= "\n" . print_r($posted,true);
    wp_mail( $to, $subject, $message );

Add that into the function you made for completed payments. If you get the email, but the order status still doesn’t update, then we know your function is being triggered, but there’s just something wrong with the logic updating the order status.

Ah, I get what you mean!
Alright, I added it and get to you back when I got something new.

I am not getting any email when I add your code into my function.

The code looks like that now:

function update_wc_order_status($posted) {
$order_id = isset($posted['invoice']) ? str_replace('ODL-','',$posted['invoice']) : '';
if(!empty($order_id)) {
$order = new WC_Order($order_id);
$order->update_status('on-hold');
$to = 'placeholder for my email, I removed it here for privacy';
    $subject = 'Email from PayPal IPN';
    $message = 'The IPN was hit';
    $message .= "\n" . print_r($posted,true);
    wp_mail( $to, $subject, $message );
}
}
add_action('paypal_ipn_for_wordpress_payment_status_completed', 'update_wc_order_status', 10, 1);

You’ve got the email code inside the if/then logic for whether or not the $order_id is empty.

Move that out so it will run any time that function is hit regardless of any other logic. You could make it the very first or very last thing in the function, for example.

Thanks for your help angelleye.