UNPROCESSABLE_ENTITY error with AUTH_FLOW_REQUIRED [closed]

We are using paypal v1 APIs, which were running fine, but from yesterday we are getting below error while trying to send invoice –

https://api.paypal.com/v1/invoicing/invoices/INV2-myInvoiceId/send. 
{
    "name": "UNPROCESSABLE_ENTITY",
    "message": "The requested action cannot be performed, and may require interaction with APIs or processes outside of the current request",
    "debug_id": "3924abcd068",
    "details": [
        {
            "field": "step_up_context",
            "value": "82f2f19af898abcdefgh95a",
            "issue": "AUTH_FLOW_REQUIRED"
        }
    ],
    "links": [
        {
            "href": "https://developer.paypal.com/docs/api/invoicing/#errors",
            "method": "GET"
        }
    ]
}

Here I am not able to understand, why it is giving error, I tried to google about ‘AUTH_FLOW_REQUIRED’ but didn’t found anything specific.
Please help me to solve it.