Koji and mock init failure

[Michael Cronenworth]

I finally have my own Koji instance ironed out, using SSL instead of 
Kerberos.

I was able to add a dist-f12 target and tags to Koji, and specified a 
localhost FTP site for a external repo. I submitted a custom SRPM I have 
as a scratch build to just test it out. The task resulted in a failure:

BuildrootError: could not init mock buildroot, mock exited with status 
20; see root.log for more information

The root.log doesn't have any visible errors. I see it did a yum install 
of the base packages. I see that /var/lib/mock has two directories (for 
my two arches: i386, x86_64) and directories and binary files inside. 
Google didn't reveal any hints either. What am I missing?