Antivirus scanner found a threat ( Win.Worm.VB-5) in the file /var/cores/core-systemd. Please delete

If it's a core dump, then it's unique to your NAS and reporting it will do no good.  It also explains why nobody else has reported it. I still think it's a false alarm, but since it's not executable, it will do no harm no matter what.  I wouldn't think the NAS would keep core dumps forever, though.  Maybe StephenB knows if they are automatically deleted periodically.