I was in the preview/alpha/whatever group. Never could get cert creation to work through the console UI, but it worked fine when using the gcloud SDK from the command-line.
Hello from the App Engine team. Could you double check that your DNS records are accurate? Everything is looking good on our end. Thanks for the feedback!
We've found the glitch in the UI and the issue will be resolved shortly. Thanks for all the support and quick testing after announcing this beta release.