I tested your scene and I’m not able to reproduce the issue. I saw that you were using Adaptive subdiv for this render, this AA method could produce weird sampling with reflective, refractive and blurry materials. I recommend you to switch to Adaptive DMC.
Long time ago I got some black spot similar to your issue, but were related to the welding option of the imported mesh and also with the UVW mapping. I don’t think this is your case, but I mentioned this just in case someone have similar issues.
Best