Seive + Spamprobe terminates with signal 6

[Mark Constable]

Ubuntu 20.04, Dovecot 2.3.7.2, SpamProbe v1.4d. For the past weeks my
sieve filters that call spamprobe have been crashing out for some users.
I have a dozen similar server setups and this is not happening on any
of the other servers and it was working just fine for a year up until
recently. This particular server is quite busy. There is plenty of disk
space and inode usage is about 2.5%. Permissions look the same as on
the other servers.

I'm hoping someone might have a suggestion as to the cause of this?

Apr 22 10:31:24 mail dovecot: lmtp(xxxx at example.com)<33016><oCuzElyQn174gAAA4xLzLg>: Error: program exec:/etc/dovecot/sieve/spamprobe (38010): Terminated abnormally with signal 6

Apr 22 10:31:24 mail dovecot: lmtp(xxxx at example.com)<33016><oCuzElyQn174gAAA4xLzLg>: Error: sieve: global: line 6: execute action: failed to execute to program `spamprobe': refer to server log for more information. [2020-04-22 10:31:24]

~ cat /etc/dovecot/sieve/global.sieve
require ["vnd.dovecot.execute", "fileinto", "envelope", "variables", "editheader"];
if header :contains "from" ["root@", "daemon@", "postmaster@"] { fileinto "Trash";
} elsif header :contains "to" ["root@", "daemon@", "postmaster@"] { fileinto "Trash"; }
if envelope :localpart :matches "to" "*" { set "lhs" "${1}"; }
if envelope :domain :matches "to" "*" { set "rhs" "${1}"; }
execute :pipe :output "SCORE" "spamprobe" ["-c", "-d", "/home/u/${rhs}/home/${lhs}/.spamprobe", "receive"];
addheader :last "X-Spam" "${SCORE}";
if header :matches "X-Spam" "SPAM*" { fileinto "Junk"; }

plugin {
   imapsieve_mailbox1_before = file:/etc/dovecot/sieve/retrain-as-spam.sieve
   imapsieve_mailbox1_causes = COPY
   imapsieve_mailbox1_name = Junk
   imapsieve_mailbox2_before = file:/etc/dovecot/sieve/retrain-as-good.sieve
   imapsieve_mailbox2_causes = COPY
   imapsieve_mailbox2_from = Junk
   imapsieve_mailbox2_name = *
   listescape_char = mail_log_cached_only = yes
   mail_log_events = delete undelete expunge copy mailbox_delete mailbox_rename
   mail_log_fields = uid box msgid size
   sieve_before = file:/etc/dovecot/sieve/global.sieve
   sieve_dir = ~/sieve
   sieve_execute_bin_dir = /etc/dovecot/sieve
   sieve_extensions = +notify +imapflags +vacation-seconds
   sieve_global_dir = /etc/dovecot/sieve/
   sieve_global_extensions = +vnd.dovecot.debug +editheader +vnd.dovecot.pipe +vnd.dovecot.execute
   sieve_max_redirects = 30
   sieve_max_script_size = 1M
   sieve_pipe_bin_dir = /etc/dovecot/sieve
   sieve_plugins = sieve_imapsieve sieve_extprograms
   sieve_quota_max_scripts = 0
   sieve_quota_max_storage = 0
   sieve_redirect_envelope_from = recipient
   sieve_vacation_default_period = 60s
   sieve_vacation_max_period = 0
   sieve_vacation_min_period = 5s
   sieve_vacation_send_from_recipient = yes
}

-- 
Mark Constable
0419 530 037
https://spiderweb.com.au