Kir

Here is the solution from my friend @halfelite <-- BIG thanks!
File /etc/default/nfs needs to be modified to add the following line:
RPC_NFSD_OPTS="-u"
This would enable NFS UDP back.
 
To make it survive the reboot, add the following line to the go file:
echo "RPC_NFSD_OPTS=\"-u\"" >> /etc/default/nfs