PROBLEM:
Click Alerts and then Actions > Manage Alert Group -> Custom Alert Group.
Then, click the + sign on the right side and pick any alert definition and press OK. Click Save and you will see the 500(server error) Server Error on the alert group screen:
And in the ambari-server.log there is error:
WARN [qtp-ambari-client-510524] ServletHandler:563 - /api/v1/clusters/<cluster-name>/alert_groups/155 java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429) at java.util.HashMap$KeyIterator.next(HashMap.java:1453) at org.eclipse.persistence.indirection.IndirectSet$1.next(IndirectSet.java:471) at org.apache.ambari.server.orm.entities.AlertGroupEntity.setAlertTargets(AlertGroupEntity.java:313) at org.apache.ambari.server.controller.internal.AlertGroupResourceProvider.updateAlertGroups(AlertGroupResourceProvider.java:344) at org.apache.ambari.server.controller.internal.AlertGroupResourceProvider.access$100(AlertGroupResourceProvider.java:60) at org.apache.ambari.server.controller.internal.AlertGroupResourceProvider$2.invoke(AlertGroupResourceProvider.java:187) at org.apache.ambari.server.controller.internal.AlertGroupResourceProvider$2.invoke(AlertGroupResourceProvider.java:184) at org.apache.ambari.server.controller.internal.AbstractResourceProvider.invokeWithRetry(AbstractResourceProvider.java:450) at org.apache.ambari.server.controller.internal.AbstractResourceProvider.modifyResources(AbstractResourceProvider.java:331)
ROOT CAUSE: https://issues.apache.org/jira/browse/AMBARI-19259
RESOLUTION: Upgrade Ambari to 2.5
