I’ve been trying to figure out why I was always getting mount.nfs: mount system call failed when trying to mount an NFS share from my Debian server on my Kubuntu box. I guess I wasn’t paying attention when dpkg / debconf configured ‘portmap’ package on the Debian box. The solution is to run dpkg-reconfigure portmap as root on the server and select no when asked:

 ┌────────────────────┤ Configuring portmap ├─────────────────────┐
 │                                                                │
 │ By default, portmap listens to all IP addresses. However, if   │
 │ this machine does not provide network RPC services (such as    │
 │ NIS or NFS) to remote clients, you can safely bind it to the   │
 │ loopback IP address (127.0.0.1).                               │
 │                                                                │
 │ This will allow RPC local services (like FAM) to work          │
 │ properly, while preventing remote systems from accessing the   │
 │ RPC services.                                                  │
 │                                                                │
 │ This configuration can be changed by editing the OPTIONS line  │
 │ in the /etc/default/portmap file and adapting the use of the   │
 │ -i option to your needs.                                       │
 │                                                                │
 │ Should portmap be bound to the loopback address?               │
 │                                                                │
 │                 <Yes>                    <No>                  │
 │                                                                │
 └────────────────────────────────────────────────────────────────┘

I hope this will help whoever else encounters the same issue.