Comment # 8 on bug 1151044 from Neil Brown

There is some issue with firewalls and NFS in the latest update.
That would not explain "Permission denied", but might explain the mount
call hanging.  So if you try again, please disable any firewall and see if that
makes a difference.

Error message concerning the failure would either come from mountd or from the
kernel.
It doesn't seem possible to enable mountd debugging with /etc/nfs.conf.
You would need to edit /usr/lib/systemd/system/nfs-mountd.service and add a 
  --debug all

to the ExecStart line.
Then 
  systemctl daemon-reload
  systemctl restart nfs-mountd

For kernel messages,
   rpcdebug -m nfsd -s all

Then try the mount.

kernel messages will appear in the output of 'dmesg'
mountd messages should appear in 
  journalctl -u nfs-mountd.service --since -1day

You can turn off debugging by reverting the change to nfs-mountd.server and
restarting the service.  For the kernel
  rpcdebug -m nfsd -c all


If you see "fsid=0" as an export option on one directory, it becomes the
directory you get when you mount "/" over NFSv4.  This is an internal
implementation detail that is now best forgotten, but some installers still set
it and some installations might have it left over from the past.  It could give
confusing results - possibly even "Permission denied" errors.