I configured PayPal Payment Acquirer in Odoo and test for payment but it is not working. Show the error page with message:

Internal Server Error

The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application. 

The flow is :

When checkout and click the pay button it redirects to the PayPal Payment page and the payment is done. And when I click the button "Return to merchant", While returning to the link /payment/paypal/dpn/ it shows this error, and this is default PayPal that comes with Odoo.

I tried on both Odoo 13 and 14, getting the same issue. How to solve this issue, can anyone help me.