Billing agreement approval URL always return error code

[msimform]

Hi,
I am trying to generate billing agreement. I can generate billing agreement successfully using REST API.
From response I get a URL for customer approval. But I always get error.

Here is generic error code: UEFZRVJfQ0FOTk9UX1BBWQ%3D%3D
Here is url for payment approval from client: https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-5CG491604D090404S

Please help me to fix this error