Hah! The service wasn't running! Trying to start it (from WebApp Admin) got more errors and I found this in the Windows Event Log (but oddly in the System section, not the Application section):

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The Visual DataFlex 15.1 Web Application Server service failed to start due to the following error:
The system cannot find the file specified.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
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This is on a fresh RC2 install BTW.

Mike