I wrote Amazon and here is their response. Looks like it's likely they'll get this fixed, hopefully.
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Hello,
I'm sorry to hear about the trouble you are having with accessing Prime Instant Videos using XBCM.
From your email, I understand that you are getting Flash DRM error while trying to access Prime instant videos using XBMC. We truly didn't expect this would happen.
This error occurred long back and has been fixed since. There appears to be some technical error due to which you are receiving this error again.
I'd love to help you, but I need some more information from you.
I request you to contact us through phone or chat so that we can try some real-time troubleshooting in an effort to resolve this problem quickly.
Contacting us through phone ensure us to have more information from you which helps us in identifying the exact problem and assist you accordingly which might fix the issue right away.
I realize that at this point of time asking you to contact us would be disappointing, however, we would really like to assist you better.
Amazon Instant Video specialists can be reached by phone or chat 24 hours a day, seven days a week using the following link:
www.amazon.com/gp/help/contact-us/digital-video.html
Thanks for your patience and, understanding. We hope you can call/chat us soon so we can help solve this problem quickly
Best regards,
Abhilash T