Looking in the individual agent logs we see that the XmlHelper class did not start up correctly:
2015-11-23 02:03:53.303 ECHO <fb8be857-fbcf-4747-973d-295a8570e581/VMwarePerformance/5.6.15/VMwarePerformance/SomeHostAgent> WARN [Quartz[0]-18488] com.quest.agent.esx.task.ESXAgentTask - Inventory and Relations - Error during FGLAM submission: Could not initialize class com.quest.glue.common.util.XmlHelper
java.lang.NoClassDefFoundError: Could not initialize class com.quest.glue.common.util.XmlHelper
Restart the Agent Manager service.