NFS: Server says "authenticated mount request", but client sees "access denied"

I've seen this if your /etc/hosts.allow and /etc/hosts.deny are not correct; check those files for a line with portmap in it and either comment it out (unsecure if you're not behind a firewall) or set the line on the client/server to be your specific subnet.

So for instance, in /etc/hosts.allow:

portmap: 192.168.0.0/16

...and comment out whatever is in /etc/hosts.deny to make only hosts.allow active. NFS uses tcpwrappers and these files to control access along with what's in /etc/exports.

your rpcinfo indicates NFS is trying to connect over UDP. it appeared NFSv4 is no longer working over UDP but is expecting TCP to be used.

the linux kernel for example is trying to mount rootfs over UDP even for the NFSv4 and needed a special argument to be added at the tail of nfsroot. example: nfsroot=192.79.143.131:/diskless/client01,tcp

I faced the same problem from a Debian 10.2 server with a macOS client. My solution:

On the NFS server, add the insecure option to the share in /etc/exports and re-run exportfs -r

Source here.