Ubuntu Server 12.04: can't write to nfs share

I have installed a small home server running 12.04, now trying to configure an NFS share with Webmin version 1.620. I have gradually lowered the security on the share to basically nil, yet any write operation to the share from any MAC OS X or Windows 7 (with NFSClient 1.7) is consistently rejected. The NFS Client on Windows e.g. returns "Permission denied" (along with lots of cryptic stuff) when trying to create a new directory, or when dropping a file.

Any hint on the matter?

Thanks a lot & best regards Mark

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