Call "HostDatastoreSystem.CreateNasDatastore" for object "ha-datastoresystem" on ESX "esxhost" failed.

When trying to setup an NFS data store you get this cryptic error message and the data store is not mounted.

The problem might be permission related, a firewall issue or similar. You can find the reason in your /var/log/vmkernel (or /var/log/messages on ESXi) which should be similar to this:

Oct  3 10:56:13 vmkernel: 4:19:06:07.879 cpu10:804820)NFS: 149: Command: (mount) Server: (nfsserver) IP: (172.16.xxx.yyy) Path: (datastore29) Label: (datastore29) Options: (None)
Oct  3 10:56:13 vmkernel: 4:19:06:07.881 cpu7:4103)WARNING: NFS: 213: Got error 13 from mount call
Oct  3 10:56:13 vmkernel: 4:19:06:07.881 cpu11:804820)WARNING: NFS: 959: MOUNT failed with MOUNT status 13 (Permission denied) trying to mount Server (nfsserver) Path (datastore29)
Oct  3 10:56:13 vmkernel: 4:19:06:07.881 cpu11:804820)NFS: 171: NFS mount nfsserver:datastore29 status: The mount request was denied by the NFS server. Check that the export exists and that the client is permitted to mount it

The solution here was to adjust the permissions of the nfs server.

We had to restart the management network after a change of DNS servers.