Hi @masterzone
I'm sorry for the late reply.
After looking at your test scene and understanding your workflow, here are my findings:
-For cases like this one, where you want to use a backplate (plane + a Corona light material) to mimic a background outdoors, you're correct by choosing to use a light material that will be applied to the plane object; however, I'll suggest disabling the "emit light" checkbox from the Corona light material instead of disabling the "cast shadows" checkbox from the Corona compositing tag.
I'm including a modified version of your scene file so you can test it and experiment with it.
I hope this helps.
Looking forward to hearing from you.
Kind regards.