I'm seeing this in yumex. I've reinstalled yum-conf-sl7x

failure: repodata/primary.sqlite.bz2 from sl-security: [Errno 256] No 
more mirrors to try.
ftp://ftp.scientificlinux.org/linux/scientific/7x/x86_64/updates/security/repodata/primary.sqlite.bz2: 
[Errno 14] curl#36 - "Offset (7638280) was beyond file size (782492)" 

http://ftp.scientificlinux.org/linux/scientific/7x/x86_64/updates/security/repodata/primary.sqlite.bz2: 
[Errno 14] HTTP Error 416 - Requested Range Not Satisfiable
http://ftp1.scientificlinux.org/linux/scientific/7x/x86_64/updates/security/repodata/primary.sqlite.bz2: 
[Errno 14] HTTP Error 416 - Requested Range Not Satisfiable
http://ftp2.scientificlinux.org/linux/scientific/7x/x86_64/updates/security/repodata/primary.sqlite.bz2: 
[Errno 14] HTTP Error 416 - Requested Range Not Satisfiable

You can try starting 'yumex -n' from a command line
and deselecting the repositories causing problems
and try again.