This article discusses an issue when a form is submitted and a HTTP 500 error is thrown with an additional error as seen below.
The required anti-forgery cookie "__RequestVerificationToken" is not present.
If the Episerver Forms packages is upgraded to newer versions there will also be a requirement to add anti-forgery token within that file's form element:
<%= Html.GenerateAntiForgeryToken(Model) %>
When the page with the form would get reloaded it would not refresh the cookie and then throw this error.