If you have a complex model with many interrelated non-manifold edges (and possibly other defects) the fix by Windows Service may take quite a while to run. I have had several take an hour or more to complete. During that time, the circle keeps spinning and the message in the window flashes as if it has crashed, but it is simply waiting for the Windows Service to respond with the “repaired” model.
I usually just open a new instance of Bambu Studio and work on something else (or go to bed and check in the morning.)