(Of course the process ID's vary.)spamd[750]: pyzor: [15578] error: TERMINATED, signal 15 (000f)
It seems the content of /var/spool/qscan/.pyzor/servers was outdated on these servers (I found 66.250.40.33:24441 and 82.94.255.100:24441 for instance), as running pyzor discover currently creates ~/.pyzor/servers as public.pyzor.org:24441.
Replacing the content of /var/spool/qscan/.pyzor/servers with public.pyzor.org:24441 seems to have helped.
Maybe this file should be automatically updated by running pyzor discover from a cronjob (as the qscand user in this case)?