More test info, starting from situation previous mail:
- unmounting ONE of the NFS datastores and recreating it again WORKS
- unmounting BOTH of the NFS datastores and recreating them again WORKS (??)
- I disconnected the Win7 client from the NFS share (and from network completely), recreating the datastores still WORKS (so much for consistency... maybe some timeout involved as well? Will wait 5 minutes now)
- I restarted the Services for NFS on the 2012R2, and recreating the datastores again WORKS
LOL, turns out I'm now UNABLE to make it FAIL 
, or so I thought:
- I rebooted the ESXi while the datastores are created: when I check in, the datastores are GONE and I'm no longer able to recreate them, this time even not when I first do an NFS connectivity test with the Win7 client
BUT this time there is another error for one of the two NFS shares (the other one still receives the error from previous message):
The specified key, name or identifier already exists:
Call "HostDatastoreSystem.CreateNasDatastore" for object "datastoreSystem-192" on vCenter Server "2008vCenter1.corpa.armaged.cu.cc" failed.
- when I first create a NEW NFS share on the 2012R2, trying to mount that one produces the original error
- however (getting more difficult all the time to see the woods through the trees), when I first create another NFS share on the 2012R2, connect to it via Win7, and THEN try to mount it, it works!
- at that point, trying to mount the two "original" datastores produces both the "other" error right above here
...
I have made an MS Network Monitor trace on the 2012R2 for both a successful and a non-successful attempt to create the NFS datastore, but that's way over my head at the moment. If anyone thinks it would be useful, I can upload