open administration -> general, select some host group for discovered hosts. delete this group in another tab. attempt to save general settings :

Error in query [update config set refresh_unsupported='600',alert_usrgrpid=7,discovery_groupid='77',severity_name_0='Not classified',severity_name_1='Information',severity_name_2='Warning',severity_name_3='Average',severity_name_4='High',severity_name_5='Disaster' where ((configid BETWEEN 000000000000000 AND 099999999999999))] [Cannot add or update a child row: a foreign key constraint fails (`zabbix_1_9`.`config`, CONSTRAINT `c_config_2` FOREIGN KEY (`discovery_groupid`) REFERENCES `groups` (`groupid`))]

there is a check for user group there, but not for the host group.

suspecting same would happen over the api.