Latest info:
Came accross https://www.softwareab.net/wordpress/?p=309 which mentions that:
"Important point on the above: the first screenshot of the storage cluster shows that Storage I/O Control (SIOC) is enabled for the datastore. This is critical: ESXi will have problems using the Windows Server 2008 R2 box as an NFS server unless SIOC is enabled"
Assuming that 2012R2 could have similar recommendation, I enabled the "Storage I/O Control" option on the NFS datastore, sadly after a reboot problem remains: datastore gone again. And after connecting with Win7 and a couple of refreshes, appears again...