NTP problems

I cannot get NTP working

I'm getting

Error
System is unable to receive time information from any NTP server. Please make sure that at least one server from the list is an active time server. Settings have not been saved.

I checked Use NTP and Continuous adjusting using NTP
ntp server tried: nt.ubuntu.nl and a variety of serverde from 0.nl.pool.ntp.org

this leads to clock skew between the NFS share and the attached server.
Model: Data Storage Software V7
Version: 7.0up11.9101.8826 64bit
Release date: 2013-09-30

Have you tried pinging this server from DSS Console?
Do you have any firewalls in between?