Magento 2.3.3 integration error: Error reading JToken from JsonReader

[nbennett25]

Using Magento 2.3.3, I've set up a sandbox Auth.net account, and added the API Login ID, Transaction Key, Public Client Key and Signature Key to the admin configuration, enabled the Payment Method and cleared the cache.

On checkout, selecting the Auth.net payment method and clicking 'Place Order', I'm getting an error displayed on the payment page:

User authentication failed due to invalid authentication values.

But I'm seeing a different error in the requests to the API:

Request URL: https://apitest.authorize.net/xml/v1/request.api
Request Method: OPTIONSStatus Code: 200 OK

Response:

{"messages":{"resultCode":"Error","message":[{"code":"E00001","text":"Error reading JToken from JsonReader. Path '', line 0, position 0."}]}}

Please advise - thx

[mmcguire]

The first error is due to an issue with the API login id or transaction key. Try re-entering them.

The second would be due to malformed JSON in the request. If you post your request (no login/transkey please) I could take a look at it.