The frontend does not match Zabbix database. Current database version (mandatory/optional): 0/0. Required mandatory version: 3000000. Contact your system administrator.
while db is:
Would be good if frontend message technically will be more correct.
p.s. Galera cluster requires 3 nodes normally as minimal number. If 2 nodes, from those 3, are offline, the single alive node will reply with the error.