[smack518]

When connection is attempted audio works fine, but video on older iOS device is black and says bad connection. iOS 11 side sees video.

I see others with this issue but haven’t found a fix.

[Ledsteplin]

Be sure you have a strong wifi/cellular signal.

Try signing out of iMessage and Facetime, then sign back in.

You might try Reset All Settings.

Go to Settings >General >Reset >Reset All Settings. This resets to default everything in Settings, including notifications, alerts, brightness, and clock settings like wake up alarms.

[smack518]

On which device? I don’t have access to the device with the older OS.

[Ledsteplin]

On which device? I don’t have access to the device with the older OS.

The one that doesn't work. If you don't have it, then the person who does needs to try my suggestions.