I had the same problem: when mounting a NFS share from the Nakivo GUI, only the NakivoBackup directory was created, and then the GUI mount procedure failed with the same error.
I solved it removing maproot/root_squash options on the NFS server. In my case it was FreeBSD 13 /etc/exports
Initial /etc/exports which prevented nakivo to set permission on files:
/vol1/nakivo1 -maproot=nakivouser -network=192.168.98.16/28
changed to the following working one
/vol1/nakivo1 -maproot=root -network=192.168.98.16/28
