Ok, I found the problem.
I use a pptp vpn connection to the kindergarten where I have my little girl.
As soon as the VPN connection is established:
sudo nmcli con up uuid ... the kindergarten's uuid ...
the log 'sshd[1548]: Received SIGHUP; restarting.' is shown.
The VPN is closed every 5 minutes so I have a script reestablishing the vpn connection
So every 5 minutes I get a new log line with 'Received SIGHUP; restarting'.
Why is sshd affected by the vpn connection?
The vpn connection configuration:
[connection]
id=some id
uuid=blablabla
type=vpn
autoconnect=true
permissions=
secondaries=
timestamp=1476446356
[vpn]
gateway=x.x.x.x
require-mppe=yes
user=xxx
password-flags=0
service-type=org.freedesktop.NetworkManager.pptp
[vpn-secrets]
password=xxx
[ipv4]
dns-search=
method=auto
never-default=true
[ipv6]
addr-gen-mode=stable-privacy
dns-search=
ip6-privacy=0
method=ignore
