It seems to be a failure on 5.1 update. I had exactly the same problem with my ATV3
There seems to be various problems with this update.
1) it won't accept a wired (Ethernet) connection; just wireless. If you're on Ethernet you,ll be able to view previews of movies in HD, YouTube, but you will lose Photostream and access to your purchases/ rents.
2)wifi will grant you back certain content: some rentals in HD, purchases. It had the "an error occurred loading this content" for an specific movie rental: "Prometheus" in 1080p.
I tried: restart,restore, everything. There are a lot of posts under this issue; some of them are proposing to restore back to 5.0.2 version using iTunes. I downloaded the ipw file from an Apple link but haven't found a way to restore it from my computer ( you can't choose a file from your computer; it just download the latest one from internet)
You can either wait for Apple for a fix or file a report on your purchase.
Note: I've seen some people with an *@apple.com mail gathering information for this issue.