hello. Same thing here. CentOS 7, upgrade from current 5.x to 6.0.
Redis was not installed before upgrade, upgrade via yum update did install zammad 6 and redis, but not enabled it. Postinstall scripts was not processed. After update got BAD GATEWAY, discover, what redis was disabled, fixed it, discover, what ticket can not be created via web, found this ticket, and solve ticket creation issue.