[Users] Error The Spamassasin plugin couldn't filter a message

[Pierre A.]

Dear Claws Mail user,

I have this error message

"Error The Spamassasin plugin couldn't filter a message.  The probable
cause of the error is an unreachable spamd daemon.  Please make sure spamd is running and accessible"

Indeed no spam message is filtered out.

The Spamassassin plugin is loaded.

In Preferences, I have
Enable Spamassin plugin
Type of transport localhost
user [me]
spamd localhost 783
Process messages on receiving
Mark spam as read
Whitelist senders found in addressbook/folder Any

spamc responds.

Other tests:

% sudo netstat -nlp|grep 783
[sudo] password for alba: 
tcp        0      0 127.0.0.1:783           0.0.0.0:*               LISTEN      24063/perl          
tcp6       0      0 ::1:783                 :::*                    LISTEN      24063/perl

% ps -ef|grep spamd
root       24063       1  0 08:03 ?        00:00:07 /usr/bin/perl -T -w /usr/sbin/spamd -d --pidfile=/run/spamd.pid --create-prefs --max-children 5 --helper-home-dir --username debian-spamd --syslog=file
debian-+   24064   24063  0 08:03 ?        00:00:08 spamd child
debian-+   24065   24063  0 08:03 ?        00:00:00 spamd child
alba       28229   14687  0 15:54 pts/0    00:00:00 grep spamd

% sudo nmap localhost -p 783
Starting Nmap 7.91 ( https://nmap.org ) at 2020-12-05 15:57 WET
Nmap scan report for localhost (127.0.0.1)
Host is up (0.00012s latency).

PORT    STATE SERVICE
783/tcp open  spamassassin

Where is my mistake?