Further to that, our Veeam installation was failing to replicate and backup with the following errors:
Error: Client error: NFC storage connection is unavailable. Storage: [stg:datastore-446,nfchost:host-6879,conn:
A similar error was produced if you tried to browse the datastores and download a file from them.
None of the solutions on Veaam or VMWare helped.
The only solution was to rebuild the Host from scratch using a 5.5 image. Of course, without vMotion live VMs cannot be migrated. But rather than shutting down and cold migrating all the VMs, just suspend them. It's only Live Migration using vMotion that fails. VMs are then only down for ~30 seconds if you are quick.