I have had the same problem for a couple of days, same error message.
The following did not solve the problem:
Authorizing and De-Authorizing my computer
Toggling Home Sharing off and on.
Signing in and out of my iTunes account on my gen 2 Apple TV
What worked: my iTunes media folders are organized, for example each movie has a separate folder. I opened the folders and found that some were missing media files (probably due to interrupted Apple TV gen 1 sync ages ago). When I double clicked such a movie in iTunes, iTunes identified it as a listing with a missing media file and marked it with an exclamation mark. It asked me if I wanted it to check for other missing files in my media folder. It found more and identified them with exclamation marks. I then unckecked all items in my iTunes list with exclamation marks. I can now stream all content from iTunes to Apple TV gen 2, no more authorization errors.
I hope this works for you too.
It would have been helpful if the error has said "Streaming via Home Sharing will not work until you uncheck items listed in iTunes that have missing media files" rather than a red herring about authorization and DRM.