Are you able to reproduce this by using the default sphere/plane scene?
My first guess would be that it could be due to the denoiser or bloom/diffraction being enabled. Second guess would be related to rendering at high resolution with the BSI file taking too long to be written for some reason. The confusing part is that you should not get a beachball, so it seems possible to be slow network or such.
To try to narrow down what may be happening, you can force stopping without denoising/bloom/diffraction/BSI by holding command down when you click the stop button.