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Basically it does not really matter if you use HTTP or HTTPS the form should work the same for both. The difference is that data served over HTTPS protocol are encrypted while for HTTP it is not.
I also tested your form with both HTTP AND HTTPS and submit every time (test make in Chrome on windows). It take a few second before it redirect though. This something that happens sometimes for long form.
If you notice the problem still persist on your hand trying clearing browser cache and also make some test on other browsers.
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