It's hard to tell the reason while there are no error logs/reports or scenario to repeat
the problem on my side.
If I did the upgrade, please contact me via email. I hope you still keep my address; otherwise use the contact form
to get back.
Currently, how do I use https for minibb? currently, it still uses unsecure http.
It's not related to miniBB, in common. It's the question to your domain holder/server provider. HTTPS means you order or purchase a SSL Certificate, at first. Depending on your choice, it could be free, like non-profit but arguable Let's Encrypt
, up to a solid and expensive Verisign, yet aligning to the middle of Comodo, GoDaddy and tons of other similar SSL providers. Having such a certificate, you or your server provider could plug it in to your domain, this will do the trick. It's not the thing which miniBB software provides.