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Actually I am going to suggest an internal reflection as well. If it was a dust donut it should be darker than the area around it. not lighter.
To add to what Bojan wrote, try reframing slightly and see if it moves around the frame, if it stays in the same place on the frame when you reposition the bright star then it is dust, if it moves, it is a reflection.
I had similar issues with my C925 (And other scopes) when using my ASI294MC camera, which had an antireflection coated sensor chamber window, not a UV/IR cut one. Fitting an IR cut filter almost completely eliminated the problem. The reflections from a star like Alnitak were spectacular without it.
I believe there is or at least used to be an IR cut window available for the 1600 cams which was not the case for the 294, so I had to use an external one.
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