The circle is a reflection of the object Plane.005, which is the empty room behind the camera. For whatever reason, the material has a Background set to black as a surface instead of a Principled BSDF, Diffuse BSDF or any other "real" object shader and I was wondering why it's not reflected as black room inside the window.
Then I looked at the glass material of the window and saw that there is a Fresnel node set to 40(!) as mix factor between a Glossy BSDF and a Transparent BSDF. This causes the reflection to somehow distort spherical between the front and back face of the glass. You can see that more clearly if you change the material of Plane.005 to white for example, you have simple reflection on the front face and the sphere on the backface - the IOR makes the reflection bend that much inside the glass.
When I set the IOR to something "normal" like 1.45, the window simply (strongly) reflects a pitch black room like a mirror, that's because there is a Glossy BSDF in there instead of a Glass BSDF. So the IOR is definitely way off. I don't know why the Archipack window has such a strange material... When I simply delete all nodes and put a Glass BSDF in there instead of this complex node setup, there is no circle and no mirrored pitch black room.