Problem:
You have mounted a shared directory on your Synology NAS using NFS. The mount succeds, but when you try to access the mount point (e.g. ls /nas) you see a Permission denied error even if running as root.
Solution:
Go into the Synology NAS web UI, go into control panel, go to shared folder edit the permissions for the shared folder you’re trying to access (right click => edit)
You likely have checked the No access checkbox for the admin user. Uncheck it, then click OK on the bottom right.
Now your NFS share should work again (even without remounting).