I have the same! I've been working on my website for a week now and have never had any problems with previewing. Now it suddenly says error 400: Font not available.
I have read all the forums before about relaunching etc and it still doesn't work.
Thank you for trying the solutions in the doc before posting here.
For all of you that are still facing this issue, please send me your Adobe ID, location, and phone number over a private message along with a link to this post. Click my picture and use the Message button to send your info. We will get in touch with you for investigation.
Alternatively, you can contact the support team directly using this link Contact Customer Care
If there is anything in your situation that is not covered in the doc, we will use the results of your investigation to update the doc.
Has this issue been solved? I just got it today when updating my site. No fonts, or anything was changed except some keywords in the metadata. What's up with this?