Hello Luke,
first try to remove ESXi host from vCenter server and add him back ... if you get the error again unmount affected datastore from that host and add ESXi back to vCenter server.
KB article which explain this issue is below:
VMware KB: Upgrading to VMware vCenter Server 5.1 or 5.5 fails with error: Datastore '<datast...
_________________________________________________________________________________________
If you found this or any other answer helpful, please consider to award points. (use Correct or Helpful buttons)
Regards,
P.