I had been experiencing the same issue, as described in comment #5. I am using Ubuntu 16.04.2 4.4.0-83, nfs-kernel-server 1.2.8-9. The fix described in comments #6 and 9 along did not work but issuing
systemctl add-wants multi-user.target rpcbind.service
service nfs-kernel-server restart
I had been experiencing the same issue, as described in comment #5. I am using Ubuntu 16.04.2 4.4.0-83, nfs-kernel-server 1.2.8-9. The fix described in comments #6 and 9 along did not work but issuing
systemctl add-wants multi-user.target rpcbind.service
service nfs-kernel-server restart
as suggested in comment #13 solved the issue.