While in the View Recordings menu and accessing your session recordings, you may experience a black screen when selecting a recording and it will fail to connect. After it attempts to connect you will see the message that the recording length is (null). This is due to your Apple TV not having enough memory to store all of your current session videos due to memory space limitations.
Depending on your Apple TV model you can typically store roughly 8 hours of session video. If you have a Split-Screen recording it will be storing each of the two videos individually which will double the stored recording length. For example, if you have a 2 hour side by side recording, your Apple TV will actually be storing 4 hours of video due to each angle having the original 2 hours of video.
To prevent loosing your session video, you can Long Press on your Apple TV Remote while over the session you wish to save and select Protect. This will prevent the Apple TV from cleaning that video out of its memory to make space for new videos.