Old NAS Enterprise modules do not have the filesystem repair feature.  If you cannot update, we recommend you boot another Linux distribution with XFS support, such as System Rescue CD:

http://www.sysresccd.org/Main_Page

After booting, your data will appear in the /dev/mapper directory (the volume designation will look like this: vg0-lv00). You'll need to mount your volumes using the mount command - please do not mount in the /mnt directory, but rather in a custom folder you have created yourself.

Afterwards, you should be able to use tools such as xfs_check and xfs_repair to diagnose the problem.