Almost once a day, the MySQL database stops accepting connections, which causes zabbix-server to stop. A 'show processlist' indicates most connection threads are attempting to update:
UPDATE ids SET nextid=nextid+1 WHERE nodeid=0 AND table_name='auditlog' AND field_name='auditid'
Doubling max_connections on MySQL doesn't help. The problem seems to occur more frequently when more users are viewing the webUI dashboard.