thanks for file. As I expected, the problem was caused by an image operator -- [BlendImages] in this case -- that didn't properly clamp the alpha-channel:
We had already fixed this bug in the current development-branch. I hope to release a new stable version within the next couple of weeks.
But you can easily fix the bug yourself:
In the shader code of [BlendImage] change at this line before the return statement at Line 52:
a= saturate(a); // <--- clamp alpha channel
return float4(rgb, a);
The scene looks really beautiful btw. One tipp though: the [StarDistort] operator actually requires two image inputs. Without these, the result is unpredicable. You can emulate the current look be using a [CheckerBoard] or [Checkerboard2] Operator.
Thanks again for reporting the issue.