Log-in fails for kubernetes-workshop: invalid_request

When I click "Log-in to access" on https://training.play-with-kubernetes.com/kubernetes-workshop/, a new browser window opens at "hub.docker-com" showing: 

```
Invalid_request
Unknown client: cc01636c-dc0d-4295-8f3a-f79e3e09bf9f
Tracking ID: ...
```

This happens with Firefox and Chromium. Any suggestions?