Same thing happened to me last week. Doing 70 mph on a bright, sunny day. The next time I started the car it was normal again. (2022 JCW Clubman)

 

This is the problem with a single camera-only system. It can be blinded. The only thing that can stop my wife's Jeep auto cruise is snow buildup on the bumper, because it uses a lower camera, upper camera, and the parking sensors in the bumper.
My '21 JCW can't engage cruise if I'm driving sunward, or if its raining too hard, or if someone is driving towards me with their brights on. It sucks because the hardware is all there, but BMW won't leverage it in favor of the pricier BMW models. Same with lane-keeping and blind spot detection - if I have working park sensors that can see a curb, why can't they see cars in the next lane?

 

I am familiar with the adaptive cruise control on our Mercedes Sprinter van and my mom's Honda CRV. Both do an excellent job even in rain or with the sun ahead. The only issue I have had is occasionally (happened 3-4 times in the last year) the Mercedes will suddenly, without reason, slam the breaks for about 1/5 of a second as if it spotted something in the road. It isn't dangerous as it is so short the speed isn't affected much but it is startling. Occasionally during heavy rain it will disengage until the rail lightens up.
Now, our 2022 JCW Countryman is a different animal. While I haven't had any real issues with it not functioning or disengaging, I have had issues with it not recognizing certain road items properly. What do I mean? Well, on the highway when you come up on a slower vehicle, the system slows you down to maintain a safe distance...normally. I have found that sometimes it doesn't recognize the vehicle ahead at all and I have to take manual control. Every time this happens, the vehicle ahead has a non-standard rear-end. Usually trailers are the culprit but even a dumptruck with unusual tail light configuration can cause it. I believe the camera is looking for the pattern of the rear of a car or truck. When it doesn't recognize it, the system ignores it. Now, I have never allowed it to get so close the auto breaking safety system might take over. I don;t think the car would ram into the vehicle ahead because of this separate system, but it is scary when it happens.
Anyone else seen this issue?

 

I have the same issue with my 2018 Countryman S. Once the cruise control says it cannot be activated, is there any way to reset it other than turn off the vehicle and then start over?

 

Yes. After the car gives you the warning and while still driving, you can toggle the cruise control to manual (non-adaptive) mode by pressing and holding the 2 buttons which control the following distance for the adaptive cruise control. After you have activated manual cruise control and (I think you have to set the speed at lease once) you can turn it off using the central button (*not* the cancel button) and then back on again. This should reactivate automatic cruise control.

 

This worked, and worked whilst the car was still running (or didn't have to turn it off and on again). Software issue?

 

Maybe? It's likely just the way it works. It requires manual intervention to re-initialize because that's what the company lawyers required. :-)