error setting up base repository

The subject message sometimes appears on the installation source entry
from anaconda. But I have never been able to find a good way to learn
just what the error is. Sure, sometimes it's obvious: the URL is wrong.
But other times everything seems to be correct but still the error
persists. I'm looking for a checklist of possible causes or a log entry
that corresponds to the problem.

In my present effort, I'm attempting to use a local mirror of the
fedora-updates repository (in addition to local mirrors of the fedora
and other repositories). I believe the mirror is correct and complete.
The machine being installed has no trouble accessing the repo from an F2
shell while anaconda is running. Looking through the anaconda logs, I
find lots of references to the updates repository but no errors.

What can cause this error message? And how do I find out what caused it
in this instance?
-- 
Dave Close