It seems that the EPiServerForms.js is not loaded correctly. Do you have @Html.RequiredClientResources("Footer"), @Html.RequiredClientResources("Header") on your template. Is there any 404 error in your browser console?
Thanks, That was the problem, a little _ had slipped in to the required....
Now it works!
We are seeing this error on a page with a simple form
It is happening here: