http > https redirect is a PITA with NGINX. I was relying on HSTS, but I've found that if you browse to the forum on a new device then it'll throw that error if you're not doing a hard redirect. Here's one way to do it using Javascript:
if (document.location.protocol === 'http:')
document.location.replace('https:' + document.location.href.substring(5));
That will fix the problem without needing to do it in the server configuration file.