Hi,

Perhaps you can check the K2 Environment Fields (i.e. Environment.FieldValue table). I have seen a few cases when the out-of-the box environment fields are not matched to the environment. 

We see the environment fields are correct

This error is pretty straightforward, but it can be difficult to figure out what's wrong. The error means the connection was terminated before any response was sent. This means a connection was established between your web server and app server, but something terminated the connection before any response was sent.

What does your setup look like? Are your smartforms and K2 running on the same machine, or are they on separate machines? If they are on separate machines, are there anything sitting between them, such as firewall, load balancer, or proxy? Do you have some sort of antivirus software running on your servers?

Did you try accessing your form or Designer directly on the server hosting smartforms? Did it return the same error?