Can't create new branch in Gitlab

You can create the branch locally and follow the commands for “Or push your current local branch from your Git clone”

Be sure to have the git origin remote set up to track the parent project’s repository. And have what you are branching from updated from its remote.

Each fork is a separate repository which does not update from the parent project. The fork does not have the 6.0.x branch since that was created in the parent project after the fork was created. That’s okay, since with both origin and the issue fork repository set up as Git remotes, you have everything available. The fork project does not need to track upstream.

Actually, be sure you are logged into git.drupalcode.org. I gave this a try, and 6.0.x is present in the fork, and the form loads as expected. GitLab usually reports 404 for access denied to avoid information disclosure issues.

Thanks @drumm, I have been merrily creating many new branches by clicking "Create new branch" for quite a while. I guess something was just not quite right between yesterday and a few hours ago ...

I have now created a fresh branch just fine ("3276799-convert-readme-file-v2"). But if you check the first branching attempt, the source is <a class="link" title="View branch in GitLab">3276799-convert-readme-file</a>, missing the href bit.

Anyway, it seems to work now, so let's keep it "Closed (works as designed)" :)