The ARC on my TV is now working again. I am not 100% sure what fixed it but the samsung tech replaced the main board (still had the problem), They then replaced the OneConnectBox (still had the problem), and then the tech called a samsung support number and they instructed us to do the steps below. I'm not sure if it was a specific one of these things that did it or if all three were required but I would suggested trying the steps below but it does not require and replacement parts.
Steps the Samsung support tech (on the phone) had us perform.
1. Turn off the Anynet+(HDMI-CEC) in the settings on the TV.
2. Turn off the CEC on the AV REceiver.
3. Power off both devices.
4. Turn on the Anynet+(HDMI-CEC) in the settings on the TV.
5. Turn on the CEC on the AV Reciever.
This is only a temporary solution. It may or may not work for a while. I bouth a samsungtv UN75RU8000 and it works for a couple of days or weeks. Then it fails. I have to do all these "ritual" to get it to work again. Today I cant get it to work at all.
Samsung should fix this from the root cause.