Can you try manually running the metadata
mirror into an empty folder? If you get a content.jar from that then you
should be able to just copy and paste it into your target.
I wouldn't overwrite the target because
you seem to have had a completely successful artifact mirror (congratulations!)
and we don't want to wait for you to get another one of those.
This is a really tough problem because
I can't seem to reproduce it. If the system wasn't trying to load the artifact
data as a metadata repository then I currently have no clue what's causing
your problem. I will continue to look into it.