I've trouble to get ntp working on our DSSv7

When I check 'use NTP' and 'Continuous adjusting using NTP' I'm getting
"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."

In the NTP servers box I've entered 0.nl.pool.ntp.org (note: I've also tried 1 through 4 taken from http://www.pool.ntp.org/zone/nl

I know NTP is working since I've configured the attached server to the dutch ntp pool as described in https://help.ubuntu.com/12.04/serverguide/NTP.html
I've also tried several ntp servers as listed by ntpq -p none of the working.

As expected GCC detected clockskew upon ompiling sources on the NFS share, confusing our users

any help would be appreciated