Indeed, I have set up HTTPS on my server, and now the plugins show up.
Next problem, apparently Glyphs can’t clone via a git+ssh:// URL …
Failed to clone repository from
git+ssh://gitserver.example.com:/home/git/glyphs/voodoo.git
to
~/Library/Application Support/Glyphs 3/Repositories/voodoo.git
Git cloning works from the command line. Maybe I should just put the repos on GitHub like you described in Using Private and Local Plugin Repositories - #20 by FlorianPircher …