As the problem is gone when the copper thickness is displayed, I suspect the error might go away if there is a minute vertical offset between the copper and the board - i.e. letting the copper plane hover above the board ever so slightly.
As the problem is gone when the copper thickness is displayed, I suspect the error might go away if there is a minute vertical offset between the copper and the board - i.e. letting the copper plane hover above the board ever so slightly.