ntpq -p reports all available servers have stratum 16. Therefor the time is cannot be set. However I can ping all the servers.
ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 master.stbuehle .INIT.          16 u    - 1024    0    0.000    0.000   0.000
 himalia.mysnip. .INIT.          16 u    - 1024    0    0.000    0.000   0.000
 x.ns.gin.ntt.ne .INIT.          16 u    - 1024    0    0.000    0.000   0.000
 services.freshd .INIT.          16 u    - 1024    0    0.000    0.000   0.000
the servers are:
server 0.debian.pool.ntp.org iburst
server 1.debian.pool.ntp.org iburst
server 2.debian.pool.ntp.org iburst
server 3.debian.pool.ntp.org iburst
Switching to different servers does not help either.
 
     
    