I had been trying to upgrade my standard edition Visual SVN Server for months now. Every time I received the email notification of a new version I would download the update, run it and the install would fail with the error "'repositories' is not a valid short filename". Google helped me find a handful of people seeing the same error, but no solutions. Ever hopeful that the next release would not have this issue, I tried again today with the latest version but the same thing happened. Then I had an idea:
Before you run the installer, open VisualSVN Server Manager and click the stop button. Then the install will complete without a hitch. Why the installer doesn't stop the service for you before it tries to install is something only the VisualSVN Team can answer.