Transactions via APMs return UNPROCESSABLE_ENTITY/TRANSACTION_REFUSED

[toster234]

I have a website where I accept payments with PayPal JavaScript SDK. Everything was working fine until around a week ago: payments via Alternative Payment Methods (notably BLIK and Przelewy24) started returning HTTP error 400 UNPROCESSABLE_ENTITY/TRANSACTION_REFUSED. Payments via PayPal work just fine.

I have confirmed that my code is correct, and this issue happens even on PayPal's example site: https://developer.paypal.com/demo/checkout/#/pattern/client

Here's the returned error when sending a POST request to https://www.sandbox.paypal.com/smart/api/order/{id}/capture (using a sandbox client ID) 

{
   "ack":"contingency",
   "contingency":"UNPROCESSABLE_ENTITY",
   "data":{
      "name":"UNPROCESSABLE_ENTITY",
      "details":[
         {
            "issue":"TRANSACTION_REFUSED",
            "description":"The request was refused"
         }
      ],
      "message":"The requested action could not be performed, semantically incorrect, or failed business validation.",
      "debug_id":"935a02fdb103",
      "links":[
         {
            "href":"https://developer.paypal.com/docs/api/orders/v2/#error-TRANSACTION_REFUSED",
            "rel":"information_link",
            "method":"GET"
         }
      ]
   },
   "meta":{
      "calc":"935a02fdb103",
      "rlog":"rZJvnqaaQhLn%2FnmWT8cSUueWscmrtUHe5Y1Bd%2FeqyvyOTq66rSXAcoHt23K4aX7r7RV0T%2BAtZVLHG%2FvY24WQLoVUuus0mS6jblFSyuJ0TX4_177caf5165b"
   },
   "server":"19jRXU8EBdP7yukvtgzHAXDIv-xYEIWzrdPNRoMdwQ_XsYAmxbAP9jIYjD0CxGyJ6IJ8hm3Fz0twTt74XStVTfN3I3K-aXMKhlCN-gf6kZf1OHYCexpTV-p6EXUdcsme0ClxWyIdFj7DkbUsSZDlYa_6n6vzE7pFKm6hcr1RWAbQ2a_CMuw-pb3Dj8KdhgpnF1oEjd8xhtuJYKURxX78H5yeAgH8n7QHaut6MhiTRBlJ7yXkhApf99ehNgyog3Rre9AXZLASc-MVBpZX66MITm"
}