Fix for the issue was to kill all backup-data-dup processes by issuing:
ps -A | grep dup
Then go to /var/lib/nethserver/backup/duplicity/backup-data/ and delete the lockfile.
We were backing up to NAS via CIFS, but we also sometimes get an error on messages file such as “CIFS VFS: Server has not responded in 120 seconds. Reconnecting…”, hence the reason why the backup would not finish even though it’s only incremental, so we had to change the Nethserver backup to NFS.