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Possibly a config setting I've missed?

Sounds like missing entries in your hobbitserver.cfg file. Add these two lines:

BBSERVERSECURECGIURL="/hobbit-seccgi" SECURECGIBINURL="$BBSERVERSECURECGIURL"

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I'm running FC3 and upgraded w/ the 4.0.3 RPM.

Unfortunately, RPM is not very good at upgrading configuration files.

That fixed it. Thanks, Henrik.