> System is a built-in account. By default, the Firebird service runs
> under that account so no user needs to be logged in for the database
> server to run. Check the properties of the Firebird service in the
> Services applet in Control Panel to see which account is being used.

Dave,
you were right. System had no rights to write in the data directory.
Of course, no one knows why; it had worked earlier on the same day.
Now it's up and running again :-)

Thanks to everybody.

Stefan