My SKP 2021 on MacOS BigSur crashes on File Save/Close every time. The log reports mention 3D Connexion, but other extensions may also contribute.
Reopened files appear to have saved correctly with changes intact.
When I run SKP 2021 with no extensions loaded I have no issues on file save/close.
I have investigated downgrading to Catalina in which had no issues but the process looks intimidating for me as I am not a sophisticated deep diver into the way the OS works.