Get error on subscribe form : errorThrown:Not Found

I get an error on the subscribe form…

Oops! There is a problem with this form:
textStatus:error
errorThrown:Not Found
responseText:

Then I changed a few things on the form and widget settings and then it started working then I changed something else on form such as adding first/last name to form and now the error has come back! Have tried to backtrack to can’t ge6t it going again.

Using a clean install of WP 4.7.3 and MailPoet 2.7.8

I’m trying to evaluate plugin, looks great except for this issue!

There are a couple of other people who have had the same issue recently and their ticket has been marked as resolved but I don’t think it was so raising it again.