mpbm23

Also in

foreach ( $woocommerce->payment_gateways()->payment_gateways() as $gateway_id => $gateway )

without ()

Have you checked your hosting.
Check your php version.
This error does not seem right and I can not reproduce it.
Can you comment out some of the code to pin point exactly the error’s origin?

I believe it has something to do with the theme and how it handles the checkout refresh. It create a lot of ajax requests.
Can you contact the theme author?

Also can you check if a note is created in the order?

Do you have SOAP installed in the server?

In line 199 of admin/class-pay4pay-admin.php add

global $woocommerce;

and in 200 replace

WC()

with

$woocommerce

Then probably the warning should not show up again.

Because subscriptions is a paid plugin and I dont have access to it can you add in the

function __construct()

$this->supports = array( 'subscriptions', 'products' );

and tell me if it works?

remove line
$.ajax( $fragment_refresh );
from woocommerce-payforpayment.php

I think that line
$.ajax( $fragment_refresh );
In woocommerce-payforpayment.php is redudant and can be deleted. But it must be first tested.

Can you try a different theme?
Also what problems does this error create except in the dashboard?

Some code that checks the wpml language can change the posid.

What referral urls did you give to Piraeus.
Why would you need a second referrer url for english?

Use

Referrer url : http(s)://yourdomain.gr/checkout-2/
Success page : http(s)://yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=success
Failure page : http(s)://yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=fail
Cancel page : http(s)://yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=cancel

I don’t believe that WPML using the parameter suffix will cause any problem?

When you disable Pay for Payment everything works correctly?

And what it has this scenario to do with the pay for payment plugin?
I think you should ask in Woocommerce support forum. Also I think that in manual orders everything is manual.