NFS to a VM problem - version issues?

I have a Kubuntu 14.04 host (vbox 4.3) with multiple Kubuntu 14.04 guests.
I use NFS (1.2.8) to communicate between the guests and the host, with no problem.

I set up test Xubuntu 18.04 host running an Xubuntu 18.04 guest and one of the Kubuntu 14.04 guests
(virtualbox 5.2.10, NFS 1.3.4).

NFS works as expected between the 18.04 host and the 18.04 guest.
If I try to mount an exported dir, it mounts.
If I try to mount an un-exported dir,
the guest gives the message:
and the host system log shows:
All correct.

But for the 14.04 guest on the 18.04 host:
if I try to mount an exported dir, I get the message:
and there is nothing in the host system log.
As though it didn't happen.

The NFS config is exactly the same between the 14.04 and 18.04 guests,
and the 14.04 guest works perfectly on a 14.04 host.

Anybody understand the problem?

I would like to migrate the hosts to 18.04 first,
then gradually migrate the guests.
But this means I need to use 14.04 guests for a while as I migrate.