Unexpected developer error, please try again later.

Took me some time to figure out, but adding cardNetworkParameters: [ { cardNetwork: "VISA" }, { cardNetwork: "MASTERCARD" }, ], to paymentDataRequest object broke the payment flow. On capacitor 3.6.0/3.9.0.

Edit: param cardNetworkParameters doesn't exist in the official docs: https://developers.google.com/pay/api/android/reference/request-objects