A Ghost Circle in the Beam of Area Lights

[FarbigeWelt]

CPU BiDir, Metropolis
Two partly overlapping area lights emit rays thru a lens. To make the beam visible there is a very flat cuboid placed in the middle of the lens. The distance of the area lights to the cuboid is the same. The cuboid is material is 0.5 Grey with 0.5 opacity.
The circle appears slightly visible at around 250 Samples and gets more visible with more samples.
Currently I have not the slightest idea where this circle comes from.
The scene is arranged as symmetric.
Whereas the circle is apparently not

A Ghost Circle

Two Area lLights, a Lense and a Ghost Cricle.zip

Two Area Lights, a Lense and a Ghost Cricle

(350.31 KiB) Downloaded 135 times

Does anybody see a reason for the existence of the Ghost Circle and its asymmetric appearance

EDIT: The Ghost Circle moves with the orthographic camera. This means in the middle of the view there is something responsible for this unintended shadow.

[B.Y.O.B.]

EDIT: The Ghost Circle moves with the orthographic camera.

Maybe some floating point precision problem.

[Fox]

It may be too rough roughness, if i recall correctly.
Let's say you have camera in pool underwater, and you want to see out of rough glass water surface, the sky, by pointing camera up, then you only see small clear circle in center (everything else is black). It's a big maybe, i most likely am wrong.