I noticed this issue starting ER5.

Plugin looks to be recognized, imported, but it can't be used. It is not even in Agent plugins. I didn't see any other messages, only log from JON server, below.

Clear, purge didn't help. Patching the old plugin didn't work either. 

LOG from JON server:

15:06:20,562 INFO  [org.apache.catalina.core.ContainerBase.[jboss.web].[default-host].[/coregui]] (http-/0.0.0.0:7080-10) org.rhq.coregui.CoreGUI PluginFileUploadServlet: file was uploaded: infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar
15:06:20,567 INFO  [org.rhq.coregui.server.gwt.PluginFileUploadServlet] (http-/0.0.0.0:7080-10) A new plugin [infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar] has been uploaded to [/tmp/upload__2de6d15d_142e344681c__8000_00000003.tmp]
15:06:20,572 INFO  [org.rhq.coregui.server.gwt.PluginFileUploadServlet] (http-/0.0.0.0:7080-10) A new plugin has been deployed [/home/hudson/jon-server-3.2.0.CR1/plugins/infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar]. A scan is required now in order to register it.
15:08:29,142 INFO  [org.rhq.enterprise.server.core.plugin.PluginDeploymentScanner] (http-/0.0.0.0:7080-12) Found plugin jar at [/home/hudson/jon-server-3.2.0.CR1/plugins/infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar] and placed it at [/home/hudson/jon-server-3.2.0.CR1/modules/org/rhq/server-startup/main/deployments/rhq.ear/rhq-downloads/rhq-plugins/infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar]
15:08:29,300 INFO  [org.rhq.enterprise.server.core.plugin.AgentPluginScanner] (http-/0.0.0.0:7080-12) Deleted an obsolete agent plugin file: /home/hudson/jon-server-3.2.0.CR1/modules/org/rhq/server-startup/main/deployments/rhq.ear/rhq-downloads/rhq-plugins/infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar
15:11:28,597 INFO  [org.rhq.enterprise.server.resource.metadata.PluginManagerBean] (http-/0.0.0.0:7080-8) Disabling plugin [10011]
15:11:28,601 INFO  [org.rhq.coregui.server.gwt.PluginGWTServiceImpl] (http-/0.0.0.0:7080-8) Disabled agent plugins: [JBoss Data Grid Server 6.x]
15:11:34,005 INFO  [org.rhq.coregui.server.gwt.PluginGWTServiceImpl] (http-/0.0.0.0:7080-8) Deleted agent plugins: [JBoss Data Grid Server 6.x]
15:11:34,039 INFO  [org.rhq.enterprise.server.core.plugin.AgentPluginScanner] (http-/0.0.0.0:7080-8) Plugin file [/home/hudson/jon-server-3.2.0.CR1/modules/org/rhq/server-startup/main/deployments/rhq.ear/rhq-downloads/rhq-plugins/jboss-datagrid-server-rhq-plugin-6.1.0.GA-redhat-1.jar] has been deleted from the file system.
15:11:41,136 INFO  [org.rhq.enterprise.server.resource.metadata.PluginManagerBean] (http-/0.0.0.0:7080-6) Scheduling plugin [AgentPlugin [id=10011, name=JDG, md5=bc3bb1cc5800b67bf516fa1cb95ccdb4]] to be purged from the database.
15:11:41,143 INFO  [org.rhq.coregui.server.gwt.PluginGWTServiceImpl] (http-/0.0.0.0:7080-6) Preparing to purge agent plugins: [JBoss Data Grid Server 6.x]. This may take a few minutes since all type definitions from the plugins must first be purged from the system. The plugins will still be visible until they have been purged. Please note that you must not re-install the plugin while the purge is running, as this is going to fail. Wait for re-add until the purge is done.
15:21:08,135 INFO  [org.rhq.enterprise.server.scheduler.jobs.AsyncResourceDeleteJob] (RHQScheduler_Worker-4) Async resource deletion - 41 successful, 0 failed, took [8378] ms
15:42:47,577 INFO  [org.apache.catalina.core.ContainerBase.[jboss.web].[default-host].[/coregui]] (http-/0.0.0.0:7080-13) org.rhq.coregui.CoreGUI PluginFileUploadServlet: file was uploaded: infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar
15:42:47,581 INFO  [org.rhq.coregui.server.gwt.PluginFileUploadServlet] (http-/0.0.0.0:7080-13) A new plugin [infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar] has been uploaded to [/tmp/upload__2de6d15d_142e344681c__8000_00000007.tmp]
15:42:47,586 INFO  [org.rhq.coregui.server.gwt.PluginFileUploadServlet] (http-/0.0.0.0:7080-13) A new plugin has been deployed [/home/hudson/jon-server-3.2.0.CR1/plugins/infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar]. A scan is required now in order to register it.
15:43:00,765 INFO  [org.rhq.enterprise.server.core.plugin.PluginDeploymentScanner] (http-/0.0.0.0:7080-13) Found plugin jar at [/home/hudson/jon-server-3.2.0.CR1/plugins/infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar] and placed it at [/home/hudson/jon-server-3.2.0.CR1/modules/org/rhq/server-startup/main/deployments/rhq.ear/rhq-downloads/rhq-plugins/infinispan-server-rhq-plugin-6.0.0.Final-redhat-2.jar]
15:43:01,051 INFO  [org.rhq.enterprise.server.core.plugin.ProductPluginDeployer] (http-/0.0.0.0:7080-13) Rediscovered agent plugin [JDG]