PaymentPayPalStandard - InvalidOperationException: Synchronous operations are disallowed

Nop 4.3 - Preinstalled Version Package only, no source
PaymentPayPalStandard installed

Hi,
I have searched the forums and Googled the problem but I am not sure how to proceed.
The error shows InvalidOperationException: Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead. when the Instant Payment Notifications (IPN) calls my site back to confirm the payment, the error is captured.

I found this article which explains how to modify the startup.cs

https://www.tektutorialshub.com/asp-net-core/asp-net-core-startup-class/

and this forum post

https://www.nopcommerce.com/en/boards/topic/81557/nopcommerce-430-rc-is-available-please-share-your-impressions/page/6

I am not a developer and I do not see how to edit the startup.cs in the installed version.

Can anyone help?  Is there a forum post that I have missed?

Thank you in advance

Please see this topic.

Hi, thank you for the suggestion, but it doesn't match the error message I am receiving.

I am not getting a 400 error, this is the message I am getting

InvalidOperationException: Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead.

https://www.simply-crafting.uk/Plugins/PaymentPayPalStandard/IPNHandler

I could be misunderstanding what I am seeing though, can anyone explain the problem?

This link https://github.com/nopSolutions/nopCommerce/issues/4870 seems to match what I am seeing, but again it talks about modifying the source and that is something I am not sure I am able to do.

Can anyone help?