Hello.
History.
2021.01 Zabbix server send alert Zabbix database is down.
I opened the log and see next

2394:20210101:104244.277 [Z3005] query failed: [2002] Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) [select a.alertid,a.mediatypeid,a.sendto,a.subject,a.message,a.status,a.retries,e.source,e.object,e.objectid from alerts a left join events e on a.eventid=e.eventid where alerttype=0 and a.status=3 order by a.alertid]
2370:20210101:104244.325 [Z3005] query failed: [2002] Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) [begin;]
2370:20210101:104244.325 [Z3001] connection to database 'zabbix' failed: [2002] Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)
2370:20210101:104244.325 database is down: reconnecting in 10 seconds

I rebooted the server and check database. Database work well, web work well.
Zabbix server health in dashboards not have problem.
Zabbix agent for localhost check mysql and show this for graph.
But now every 15 min I got message for email Zabbix server database is down.
I set debug level 4 but I not see error message.
I create dump and open on dump on new database(zabbix2).
I did Trancate table history,history_str,history_uint,history_log,history_text and did mysqlcheck, restart zabbix-server use zabbix2 database.
But nothing changed, every 15 min message Zabbix database is down.

Why I get this messages if database work well ?
What logic uses the zabbix-server what make conclusion Zabbix database is down ?

Thanks.