As @lah7 says this community, which handles authentication credentials, is only available via https.
Originally the main ubuntu-mate.org site was also https only too, but Windows XP user (Yes, I know) couldn’t download the iso images etc. So the main site can be accessed via http too. But I’m going to be switching back to https only for all sites in a couple of months. Just wanted to give some XP hold outs time to download and migrate to Ubuntu MATE 
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