you could try to remove that database file. It should be recreated on service start automatically.
systemctl stop adguardhome.service
mv /mnt/dietpi_userdata/adguardhome/data/sessions.db /mnt/dietpi_userdata/adguardhome/data/sessions.db.save
systemctl start adguardhome.service
journalctl -u adguardhome.service