at 10 NAC's, all within the same subnet the customer register time differences from some minutes to 8 hours.
on tuesday these NAC's are updated from version 5.0 to 5.1 and rebootet. 
the following NAC config is from the installation wizzard and the NTP server reacheble...
I need some hints how to debug this and how to find the source of the problem...
# Sample
/etc/ntp.conf: Configuration file for ntpd.    #
    #  Undisciplined Local Clock. This is a fake driver intended for backup
    # and when  no outside source of synchronized time is available. The
    # default  stratum is usually 3, but in this case we elect to use stratum
    # 0. Since  the server line does not have the prefer keyword, this driver
    # is never  used for synchronization, unless no other other
    #  synchronization source is available. In case the local host is
    #  controlled by some external source, such as an external oscillator or
    # another  protocol, the prefer keyword would cause the local host to
    # disregard  all other synchronization sources, unless the kernel
    #  modifications are in use and declare an unsynchronized condition.
    #
    fudge  127.127.1.0 stratum 10
    #server  pool.ntp.org
     
    #
    # Drift  file. Put this in a directory which the daemon can write to.
    # No  symbolic links allowed, either, since the daemon updates the file
    # by  creating a temporary in the same directory and then rename()'ing
    # it to the  file.
    #
    driftfile  /etc/ntp/drift
    multicastclient  # listen on default 224.0.1.1
    broadcastdelay  0.008
     
    #
    # Keys  file. If you want to diddle your server at run time, make a
    # keys file  (mode 600 for sure) and define the key number to be
    # used for  making requests.
    # PLEASE DO  NOT USE THE DEFAULT VALUES HERE. Pick your own, or remote
    # systems  might be able to reset your clock at will.
    #
    #keys  /etc/ntp/keys
    #trustedkey  65535
    #requestkey  65535
    #controlkey  65535
     
    # Don't  serve time or stats to anyone else by default (more secure)
    restrict  default noquery nomodify
    # Trust  ourselves. š
    restrict 127.0.0.1
     
    #ntp enabled
    server 10.20.99.222
server 10.20.99.223