Upgraded from 3.4.1 to 3.4.2. DB upgrade completes but Server fails to connect to DB.

1040:20171002:091617.102 Zabbix Server stopped. Zabbix 3.4.1 (revision 71734).
17484:20171002:091617.146 Starting Zabbix Server. Zabbix 3.4.2 (revision 72885).
...
17484:20171002:091617.146 using configuration file: /etc/zabbix/zabbix_server.conf
17484:20171002:091617.172 current database version (mandatory/optional): 03040000/03040000
17484:20171002:091617.172 required mandatory version: 03040000
17484:20171002:091617.172 optional patches were found
17484:20171002:091617.172 starting automatic database upgrade
17484:20171002:091617.422 completed 100% of database upgrade
17484:20171002:091617.422 database upgrade fully completed
...
17646:20171002:091619.325 [Z3001] connection to database 'zabbix' failed: [1040] Too many connections
17636:20171002:091619.326 [Z3001] connection to database 'zabbix' failed: [1040] Too many connections
17641:20171002:091619.326 [Z3001] connection to database 'zabbix' failed: [1040] Too many connections
17631:20171002:091619.326 [Z3001] connection to database 'zabbix' failed: [1040] Too many connections
17616:20171002:091619.326 [Z3001] connection to database 'zabbix' failed: [1040] Too many connections
17640:20171002:091619.326 [Z3001] connection to database 'zabbix' failed: [1040] Too many connections
17634:20171002:091619.326 [Z3001] connection to database 'zabbix' failed: [1040] Too many connections
17646:20171002:091619.332 Cannot connect to the database. Exiting...
17636:20171002:091619.334 Cannot connect to the database. Exiting...
17641:20171002:091619.336 Cannot connect to the database. Exiting...
17631:20171002:091619.337 Cannot connect to the database. Exiting...
17616:20171002:091619.339 Cannot connect to the database. Exiting...
17640:20171002:091619.340 Cannot connect to the database. Exiting...
17634:20171002:091619.341 Cannot connect to the database. Exiting...

After several attempts to correct the issue I had to roll back to 3.4.1 by performing a yum history undo.

Once the 3.4.1 was installed I was able to start the service and connect to the server.

I have the same issue one my two proxy servers which also run RHEL 7.4

2754:20171002:093428.504 Starting Zabbix Server. Zabbix 3.4.1 (revision 71734).
2754:20171002:093428.504 ****** Enabled features ******
2754:20171002:093428.504 SNMP monitoring: YES
2754:20171002:093428.504 IPMI monitoring: YES
2754:20171002:093428.504 Web monitoring: YES
2754:20171002:093428.504 VMware monitoring: YES
2754:20171002:093428.504 SMTP authentication: YES
2754:20171002:093428.504 Jabber notifications: YES
2754:20171002:093428.504 Ez Texting notifications: YES
2754:20171002:093428.504 ODBC: YES
2754:20171002:093428.504 SSH2 support: YES
2754:20171002:093428.504 IPv6 support: YES
2754:20171002:093428.504 TLS support: YES
2754:20171002:093428.504 ******************************
2754:20171002:093428.504 using configuration file: /etc/zabbix/zabbix_server.conf
2754:20171002:093428.516 current database version (mandatory/optional): 03040000/03040001