When clicking the button in ka72.com, the post is sent to
gpsteamchallenge.com.au/sailor_session/post which is then redirecting to
gpsteamchallenge.com.au/user/login which then, for some reason redirects again to
gpsteamchallenge.com.au/user/login (note this is a different url, http not https) for the actual login, which means that the user is logging in to the http version of the site, not the https version.
I am seeing similar weird behaviour if I just go direct to
gpsteamchallenge.com.au/user/login and then log in, I get sent to an http page which says "The information you're about to submit is not secure Because this form is being submitted using a connection that's not secure, your information will be visible to others."
I'm not sure I can do anything to fix this behaviour at my end. The redirect seems to be happening from the https version of the page at gpsteamchallenge to the non-https version of the page.
I also noticed this message in my Chrome console:

I've seen this happen on old pages that have been moved to https but still contain references to full URLs. In this case, you can usually drop the "http:" from the url entirely and the browser will use the correct protocol for the circumstances.
Dylan