It appears that this issue is related to specific network/Internet configurations. If you are experiencing this issue (some features work over WiFi but not Ethernet) after updating to Apple TV version 5.1, please send me <email@example.com> an email with this information:
1. What is the manufacturer, model number and firmware version (if possible) of your Ethernet router?
2. What is the manufacturer, model number and firmware version (if possible) of your WiFi router? (if it is different)
3. What is your Internet service provider?
4. Where are you located geographically? (what city?)
5. What Internet features do work over Ethernet?
6. What Internet features do not work over Ethernet?
7. Does it read "Activation Failed" at the bottom right in Settings > General > About?
Please make the subject line of your email be "Some features work over WiFi but not Ethernet".