THX for the response, Shane.
Regarding the failed up date on the Windows machine: I'd used the "Update" menu item to start the update process. When the 12.0.1 update showed up as available I started the update. That specific update process includes a stop point telling me to quit AI - which I did at that point. Then the process failed as in the original post to this thread.
The Mac update fails (the same way) on two machines (both licensed of course). I haven't moved any files inside the AI application folder for either of these installations.