If you get this error:

Mar 17 23:56:14 mysql01 mysqld: 2019-03-18T03:56:14.176594Z 4770 [Note] Access denied for user 'zabbix'@'mysql03.nix.mds.xyz' (using password: YES)

Verify your password in the following two files:

[root@linsrvj01 zabbix]# cat /etc/zabbix/web/zabbix.conf.php
<?php
// Zabbix GUI configuration file.
global $DB;

$DB[‘TYPE’]     = 'MYSQL';
$DB[‘SERVER’]   = 'mysql-c01';
# $DB[‘PORT’]     = '0';
$DB[‘PORT’]     = '3306';
$DB[‘DATABASE’] = 'zabbix';
$DB[‘USER’]     = 'zabbix';
$DB[‘PASSWORD’] = 'BADPASSWORD';

// Schema name. Used for IBM DB2 and PostgreSQL.
$DB[‘SCHEMA’] = '';

$ZBX_SERVER      = 'localhost';
# $ZBX_SERVER      = 'linsrvj01.nix.mds.xyz';
$ZBX_SERVER_PORT = '10051';
$ZBX_SERVER_NAME = 'ZabbixLAB';

$IMAGE_FORMAT_DEFAULT = IMAGE_FORMAT_PNG;
[root@linsrvj01 zabbix]#
 

and the following file:

[root@linsrvj01 zabbix]# cat /etc/zabbix/zabbix_server.conf|grep -v "#"|sed -e "/^$/d"
LogFile=/var/log/zabbix/zabbix_server.log
LogFileSize=0
PidFile=/var/run/zabbix/zabbix_server.pid
SocketDir=/var/run/zabbix
DBHost=mysql-c01.nix.mds.xyz
DBName=zabbix
DBUser=zabbix
DBPassword=BADPASSWORD
DBPort=3306
SNMPTrapperFile=/var/log/snmptrap/snmptrap.log
Timeout=4
AlertScriptsPath=/usr/lib/zabbix/alertscripts
ExternalScripts=/usr/lib/zabbix/externalscripts
LogSlowQueries=3000
[root@linsrvj01 zabbix]#
[root@linsrvj01 zabbix]#
 

and change accordingly to ensure it's correct.  Likewise, ensure permissions are sound:

mysql> uninstall plugin validate_password;
ERROR 1305 (42000): PLUGIN validate_password does not exist
mysql> GRANT ALL PRIVILEGES ON *.* TO 'zabbix'@'%.mws.mds.xyz' IDENTIFIED BY 'zabbix' WITH GRANT OPTION;
Query OK, 0 rows affected, 1 warning (0.02 sec)

mysql> GRANT ALL PRIVILEGES ON *.* TO 'zabbix'@'%.mws.mds.xyz' IDENTIFIED BY 'zabbix' WITH GRANT OPTION;
Query OK, 0 rows affected, 1 warning (0.53 sec)

mysql> GRANT ALL PRIVILEGES ON *.* TO 'zabbix'@'%.nix.mds.xyz' IDENTIFIED BY 'zabbix' WITH GRANT OPTION;
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> flush privileges;
Query OK, 0 rows affected (0.01 sec)

mysql> INSTALL PLUGIN validate_password SONAME 'validate_password.so';
Query OK, 0 rows affected (0.01 sec)
mysql>
 

Then restart the front end:

systemctl restart httpd zabbix-server zabbix-agent;

Enjoy!

Thx,
TK