replacing with debug flag, this works through reboot without any restart required in rc.local
ExecStart=/usr/sbin/rpc.nfsd -d
RPCNFSDARGS was not defined in /etc/default/nfs-kernel-server
and it wasn't read from there when set - defining it as "-d" did not enable the option or allow the service to start.
system updated and problem returned in 4.4.0-31
fixed by removing $RPCNFSDARGS from line 29 in
/lib/systemd/ system/ nfs-server. service
replacing with debug flag, this works through reboot without any restart required in rc.local
ExecStart= /usr/sbin/ rpc.nfsd -d
RPCNFSDARGS was not defined in /etc/default/ nfs-kernel- server
and it wasn't read from there when set - defining it as "-d" did not enable the option or allow the service to start.