You may see error “-6175, 0”; connectivity or security errors may pop up; but definitively you will notice that the “Quickbooks database service” will not start.
The solution given by a QuickBooks tech just now, is to change the user under which the database service runs, from the special QuickBooksDBxx service user, to the Local System. I asked if there was any reason not to do this under any circumstances, and he said there is not. This seems to make QuickBooks as a whole run better. It is needful to give “SYSTEM” full rights to any folder containing QuickBooks files.