Plugin PayPalStandard response Handler 400 Bad request Nop version 4.50

Hi,  it is the first time that I use the PayPal Standard plugin, I have already configured the settings for the plugin and I am using SandBox mode, but after making the payment on the Paypal side when I click the "Return to Merchant" button from paypal It throws me the Bad Request 400 error. for the return site Plugins/PaymentPayPalStandard/PDTHandler,

I have seen other forums here about this problem but it seems that there is no specific solution or if there is?

Used Nopcommerce 4.50

I have got the same issue, System log shows errors with the following:
Log level Error
Short message Error 400. Bad request
Full message
IP address x.x.x.x
Customer Guest
Page URL https://xxxxxx/Plugins/PaymentPayPalStandard/PDTHandler
Referrer URL https://www.sandbox.paypal.com/

In the browser (firefox) used I see in developer mode the following "Cookie ‘.Nop.Customer’ is geweigerd omdat deze al is verlopen." something like the mentioned cookie is expired

I also have this problem - did you sort it out ?

https://localtest45.selectsystems.com.au/Plugins/PaymentPayPalStandard/PDTHandler

This page isn’t working
If the problem continues, contact the site owner.
HTTP ERROR 400

Yidna,
When I put your URL in the browser address bar, I get

This site can’t be reachedCheck if there is a typo in localtest45.selectsystems.com.au.
If spelling is correct, try running Windows Network Diagnostics.
DNS_PROBE_FINISHED_NXDOMAIN

Is your URL publicly accessible?

Thanks, Sorry I entered the wrong URL.
https://v45demo.selectsystems.com.au/Plugins/PaymentPayPalStandard/PDTHandler

But on checking again it seems to be a problem using Sandbox and Test Credentials
Works Ok when I use my real merchant account