Authentication failed for zabbix

[typedef75]

Hello,
my environment:
1x zabbix_server 4.0.10 (MySQL)
6x zabbix_proxy 4.0.10 (MySQL, No encryption)

I've founded in zabbix_proxy.log of only one of my zabbix_proxies following generated messages:
Authentication failed for zabbix
Authentication failed for zabbix
Authentication failed for zabbix

I disabled in zabbix gui all hosts monitored by this zabbix_proxy, restarted zabbix_proxy, but the messages are still generated in zabbix_proxy.log. Thera are no relevant (concerned) messages in zabbix_server.log.

Any idea what this message means?

Thanks a lot.