Thank you for your replay, but this is not working for me. the last backup log is from Aug 2018 and even after deleting the content of the folder /var/lib/nethserver/backup/duplicity/ the backup fails.
Jan 14 23:43:01 home2 systemd: Started Session 8459 of user root.
Jan 14 23:45:01 home2 systemd: Created slice User Slice of apache.
Jan 14 23:45:01 home2 systemd: Started Session 8460 of user apache.
Jan 14 23:45:08 home2 systemd: Removed slice User Slice of apache.
Jan 14 23:45:08 home2 sssd: tkey query failed: GSSAPI error: Major = Unspecified GSS failure. Minor code may provide more information, Minor = Server not found in Kerberos database.
Jan 14 23:45:09 home2 sssd: tkey query failed: GSSAPI error: Major = Unspecified GSS failure. Minor code may provide more information, Minor = Server not found in Kerberos database.
Jan 14 23:45:09 home2 sssd: tkey query failed: GSSAPI error: Major = Unspecified GSS failure. Minor code may provide more information, Minor = Server not found in Kerberos database.
Jan 14 23:45:09 home2 sssd: tkey query failed: GSSAPI error: Major = Unspecified GSS failure. Minor code may provide more information, Minor = Server not found in Kerberos database.
Jan 14 23:45:10 home2 sssd: tkey query failed: GSSAPI error: Major = Unspecified GSS failure. Minor code may provide more information, Minor = Server not found in Kerberos database.
Jan 14 23:47:54 home2 sshd[21928]: Connection closed by 146.66.244.246 port 42014 [preauth]
Jan 14 23:50:01 home2 systemd: Created slice User Slice of apache.
Jan 14 23:50:01 home2 systemd: Started Session 8461 of user apache.
thats all I found I just deletet some dhcp messages
Jan 14 03:00:04 mail esmith::event[4239]: Action: /etc/e-smith/events/pre-backup-data/S20nethserver-backup-config-predatabackup SUCCESS [1.65277]
Jan 14 03:00:04 mail esmith::event[4239]: Action: /etc/e-smith/events/pre-backup-data/S50mysql-dump-tables SUCCESS [0.362955]
Jan 14 03:00:04 mail esmith::event[4239]: Action: /etc/e-smith/events/pre-backup-data/S50nethserver-ejabberd-backup SUCCESS [0.297388]
Jan 14 03:00:04 mail esmith::event[4239]: Action: /etc/e-smith/events/pre-backup-data/S50nethserver-webtop5-backup SUCCESS [0.152889]
Jan 14 03:00:04 mail esmith::event[4239]: Event: pre-backup-data SUCCESS
Take the number in parentheses like [4239] and then launch
ls -l /etc/e-smith/events/actions/pre-backup-data
ls: Zugriff auf /etc/e-smith/events/actions/pre-backup-data nicht möglich: Datei oder Verzeichnis nicht gefunden
(Access to /etc/e-smith/events/actions/pre-backup-data not possible: file not found)