[CentOS] yum grouplist error

I keep a local copy of the yum repository using rsync every night.   
Commands like "yum update" still work, but lately I've been getting  
the following error:

   # yum grouplist
   Setting up Group Process
   Setting up repositories
   Error: Caching enabled and local cache: //var/cache/yum/base/ 
yumgroups.xml does not match checksum

My nightly rsync cron job doesn't update this file -- should it?  How  
does one rebuild it?  To set up my local repository, I edited the  
file /etc/yum.repos.d/CentOS-Base.repo and changed all baseurl  
entries to use file:///path/to/my/repo instead of http:// 
mirror.centos/org/...  Do I have something misconfigured?

Thanks,
Alfred