Asustor appears to have this problem and does not work with XBMC NFS or a few other things which use NFS, you can manually edit the exports file adding the insecure flag on the NAS but thats not very user friendly.6.1 Troubleshooting NFS on a commercial NAS with XBMC
Your NFS server on your NAS needs to be able to allow connections on so-called unprivileged ports, which are port numbers higher than 1023. However, most NAS's are set up by default to deny incoming NFS connections on these unprivileged ports. This is one of the most common reasons why XBMC users experience problems when trying to connect to an NFS shared folder on a NAS.
If you are having trouble connecting to your NFS shares on your NAS from XBMC, make sure your NFS shares have the "insecure" option enabled, which will allow connections on unprivileged ports. To check, connect to your NAS via ssh and issue the cat /etc/exports command. Your shared folders will each have several parameters listed separated by commas. If the "insecure" parameter is not listed, you will need to add that parameter to the list. This will usually involve directly editing the /etc/exports file with a UNIX text editor and adding the "insecure" option manually, although some NAS's may offer the ability to add the option via an admin interface. Check your NAS manual for details, or explore the section below if your brand of NAS is listed.
Please add an allow connections from unprivliged/insecure ports to the NFS privileges window which would fix this and make NFS a viable option.
Thank you.