Must I register again ? Because you changed domains or just use the same login info ?
You don't need to register again, you can use your same log-in details to continue browsing that forum.
The site is secured you can change the url address from http to https.
Admin should redirect it to the https url, I checked the site and it is indeed in https connection. The problem I see why http is seen is the link provided by the one who posted the site but it should be fixed by redirecting http connection to https since https is also available for the site.