[CentOS] yum update error

Hi folks

bit of an odd one
over the last few days i have been getting

[Errno 14] HTTP Error 500: Internal Server error
Could not retrieve mirrorlist
http://mirrorlist.centos.org/?release=5&arch=i386&repo=updates error
was
[Errno 14] HTTP Error 500: Internal Server Error

(though not just with updates but any mirrorlist from CentOS-Base and
also fasttrack)

this happens when trying to do a yum update

when i try browsing to the mirrorlist address manually i get

Sorry, the requested URL
'http://backend/?ra=xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx&&release=5&arch=i386&repo=os'
caused an error:

the x'd portion represents the boxes network address from my "provider
independent" ipv6 space which i recently moved to
I am dual-stacked

everything else is working and cached mirrors appear to work ok
using the baseurl instead also works but isn't something i want to do for long

any ideas?

mike