A little bit forward.
First, I changed the authetification type.
Second, in the addon configuration, I switch from 0.0.0.0 to 127.0.0.1.
Now, I have the message saying that kodi is unable to reach the server.
Please find below the new kodi.log.
In the /etc/default/tvheadend file, I changed the log value from 0 to 1 (log to syslog).
I joined also the tvheadend extract from syslog.
According to your screen copy, the limit connection is 10. Mine is 0. So I changed to 10 but nothing more...