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Looks like maybe you have overlapping polygons -- one black, another grey? Render with the debug utility shader using a primitive or object mode and it might show the problem more clearly. If this is the issue, you need to fix your geometry.
Yes, I've had this issue before with bevels. Try bevelling it differently (reverse the surface/curve direction).
I fixed it this way: 1. Ignore Subdivision in Diagnostics in Render settings 2. triangulate object. thanks for help)