I changed the configuration on utca6:

# You do need to talk to an NTP server or two (or three).
#server 131.169.40.65
#server 131.169.194.85
#server 131.169.194.86
server 10.131.169.100

and the new ntp configuration on 18911 is:

#server 131.169.40.65
#server 131.169.194.85
#server 131.169.194.86
server ntp.ubuntu.com
server ptbtime3.ptb.de
server pool.ntp.org

# Use Ubuntu's ntp server as a fallback.
server 127.127.1.0

# Local users may interrogate the ntp server more closely.
restrict 127.127.1.0
fudge 127.127.1.0 stratum 13

# Clients from this (example!) subnet have unlimited access, but only if
# cryptographically authenticated.
restrict 10.131.169.0 mask 255.255.255.0


# If you want to provide time to your local subnet, change the next line.
# (Again, the address is an example only.)
broadcast 10.131.169.255

# If you want to listen to time broadcasts on your local subnet, de-comment the
# next lines.  Please do this only if you trust everybody on the network!
disable auth
broadcastclient