That seems like an odd submission URL, and indeed, I can see that in our database your “action” parameter is set to “POST” rather than a path, which is the intention. Method must == POST, definitely! But action should be a path at which we’ll accept a POST, and at all other paths, we will 404 on POST requests. I’d suggest at least qualifying it (/POST) if that is your intention - but I don’t feel like it is.
Here’s some more details on how our form submissions work and how to debug them, which would be good reading as you troubleshoot since my spidey sense suggests that you have a misbehaving html representation of that form somewhere in your deploy and as mentioned in this post, the HTML file is how we set up our system to be ready to receive your POSTs 
If that doesn’t help, do please let me know the URL for your html file that contains the form definition (I think it must be somewhere else than the version you linked to, to have that action=POST situation…)