It’s the “black highlights” issue described here - https://support.chaos.com/hc/en-us/articles/4528311915025-Known-bugs-and-limitations
There is a workaround shared in that article.
Another option is not using backplate objects at all or replacing them with environment overrides.
Hi,
This looks like a infamous “black highlights” bug. It’s a well known issue documented in Corona help portal. You can find the cause of this issue and possible workarounds here: https://support.chaos.com/hc/en-us/articles/4528311915025-Known-bugs-and-limitations
Damn, beaten by the Kid a fast draw :]
After so many weeks of struggle, I finally found a solution — thank you!!
In my case, the issue was usually the background, which had “cast shadows” enabled.
Now, when I disable that option, it unfortunately starts casting a shadow. Is there any way to work around this?
I thought that disabling it from reflections would help, but then the view no longer shows up in the windows. 
Did you try workaround provided in this topic, i.e. using rayswitch material? https://forum.corona-renderer.com/ geosphere as background #msg181607
Can this limitation be solved?
Not easily, no, sorry - from a technical standpoint it would be very hard and very time consuming with a risk of breaking a great many other things along the way (else it would have been solved already).
You should use a Corona Light Material or (optionally a Corona Physical Material with self-illumination enabled) rather than a Corona Physical Material with a diffuse color.
Then please follow the instructions from this post: https://forum.corona-renderer.com/ geosphere as background #msg181607
And also make sure not to disable cast/receive shadows in object properties of background plane object - Corona rayswitch material should handle this if properly set up.
Do u have a glass object?
have you tried connect it on Refract override instead ?
You have the white Light Material used as direct and refract visibility override.
