Issue #3263942 by AndyF: The payment remote ID is not a string until saving

src/Plugin/Commerce/PaymentGateway/Onsite.php

@@ -151,7 +151,7 @@ class Onsite extends OnsitePaymentGatewayBase implements OnsiteInterface {
       // The remote ID returned by the request.
       $next_state = $capture ? 'completed' : 'authorization';
       $payment->setState($next_state);
-      $payment->setRemoteId($responseXml->ssl_txn_id);
+      $payment->setRemoteId((string) $responseXml->ssl_txn_id);
       $payment->save();
     }
     else {