Apologies Mr. Takayuki. It’s not possible to give you access to the form page without logging in. It is possible to access the API URL I posted at this time, as subscribers_only is not turned on right now.
Do you have any ideas as to what the problem might be with the API endpoint not recognizing that the browser from which the call is sent is actually logged in at that time? As mentioned above, I’ve also implemented the nonce filter you’ve posted, and the form on the page is working perfectly under all situations.
Thank you for your help!