Currently for NFS, in Webmin configuration pages, exporting NFS directories with NFS 4.0 protocol doesn't work, i.e., clients could not mount those directories. However, with NFS 3.0 protocol, it does work. Very low priority.
Please give more detailed info is this issue iwht Server NFS or client attachment or Webmin configuring.
Webmin configuration. When exporting any directory in webmin, webmin can't fulfill that and will reports error. So, for the client, it can't mount for sure.
Fixed by Kevin and merged in meta-baryon revision aaf25b930c193188f39cd5aa9e80c4061570456c.