Problem solved, though it sounds very odd to me....
I saw a post somewhere that discussed restarting FileMaker "service".
So I tried to close the Sample Database, so I could then restart the service using the Services control panel. But the Close Database command refused to function. So I tried stopping the WPE and that worked. Then I tried stopping the Database Server and that failed. So, I thought, who cares about corruption in the Sample Database, so I just used the Windows Control Panel to stop the service and restart it.
And now, I can upload my database.
BTW -- is there any recommendation about updating Java to version 7?