Angus, na versão mais recente, há um problema que pode impedir o TC de funcionar (3.5.0.beta2-dev, Theme component have become nonoperative due to the discourse-events plugin).
Uncaught Error: Could not find module `discourse/mixins/singleton` imported from `discourse/plugins/discourse-events/discourse/models/provider`
at loader.js:247:1
at h (loader.js:258:1)
at u.findDeps (loader.js:168:1)
at h (loader.js:262:1)
at u.findDeps (loader.js:168:1)
at h (loader.js:262:1)
at requireModule (loader.js:24:1)
at y (app.js:170:18)
at b (app.js:193:19)
at app.js:156:29
at g.start (app.js:167:1)
at HTMLDocument.<anonymous> (start-app.js:5:7)
at discourse-boot.js:13:12
at discourse-boot.js:1:1
Acredito que esteja relacionado a esta alteração, onde o mixin foi refatorado para um decorador de classe: DEV: refactor singleton mixin to class decorator by tyb-talks · Pull Request #30498 · discourse/discourse · GitHub.