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.