Yesterday I just did a clean installation of Cygwin 1.7.20-1 and I have experienced exactly the same issue as you - mountd and nfsd don't start (get_myaddress: ioctl: Invalid argument in logs). Portmap starts fine.
I tried to downgrade sunrpc, but that didn't help.
I would like to report that this is still an issue with the latest release of cygwin. (Cgywin1.dll 1.7.25) I can confirm that the NSFD and MOUNTD services for the nfs server will die within 10 minutes after starting. When i rolled back to Cygwin1.dll 1.7.18 this problem no longer exists.
I had an old cygwin installation. Though I believe you can pull some archive installations off old mirrors. I saw another thread that suggested as much.