After a reinstallation with the last libreelec version (before i had a double boot with ubuntu and an openelec upgraded to libreelec), i found impossible adding nfs sources as i used to, simply using "add source" - "nfs"
It worked perfectly with my previous installation, and the NFS server works perfectly as i use it from other linux machines.
I finally got access to my nfs server adding nfs mounting scripts with ssh as described here https://kodi.wiki/view/nfs#nfs_sharing_from_linux
Actually, i still didn't understand the reason why the previous behaviour of simply adding nfs source from libreelec interface didn't work...
This post is to share the solution i used, and if it's possible, also to get an explaination, or opening a bug thread
Good night all.