Well, so much for crossing fingers.. I make a snapshot of ESXi3, and right after that I find that I lost connection to the datastore again. 
Next episode: I revert to the ESXi3 snapshot, at that moment the NFS datastore briefly re-appears, but after a "refresh" it's gone again. Now I connect the Win7 client to another NFS share on the 2012R2 server, when I try to re-create the datastore on ESXi3 after that, it comes back with the "already exists" error. After a refresh, the datastore appears, uhg????? Which seems to indicate that connecting with ANOTHER NFS client to the NFS server resurrects the NFS datastore on the ESXi3 host???
After another reboot, it's still there and I can successfully upload a 600MB+ folder.
Still seems I better only store some ISOs there for the moment, lol...