I read that thread also.
I think it's not exactly the same problem. She mentioned artifacts that appear in areas where surfaces face each other in a 90 degree angle. BB mentioned this is already known as bug. You see this in my render also, but the effect is much more obvious and it seems to depend on whether there is something below the light or not. But maybe the issues are related to each other. Let's see. I think I should send a bug report.
By the way, I am not advanced. I am still asking beginners stuff here all the time.