Then Vikunja is only running on http (port 80). To make it available on https (port 443), you will need to host it with https on your server. Cloudflare can only help you there in that it can give you a valid tls certificate for free but it can’t make a site you’re hosting on http available via https.
Check out this thread for information on how to do this with a self-signed certificate:
I was also a complete newbie when I started exploring Vikunja.
I understand what you’ve been through.
So I rewrote the steps I did successfully after a series of failed tests.
This is a collection from tutorials on the internet, tweaked to make it work in my case.
Hope it helps you: