I found /usr/local/psa/var/log/maillog
files filled with the following error message on several servers with pyzor (and qmail-scanner) installed:
spamd: pyzor:  error: TERMINATED, signal 15 (000f)
(Of course the process ID's vary.)
It seems the content of /var/spool/qscan/.pyzor/servers
was outdated on these servers (I found 184.108.40.206:24441 and 220.127.116.11:24441 for instance), as running pyzor discover
currently creates ~/.pyzor/servers
Replacing the content of /var/spool/qscan/.pyzor/servers
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)?