OBR plugin installation errors on Confluence 5.6.1

2014-09-06 10:47:28,854 WARN [UpmAsynchronousTaskManager:thread-2] [upm.core.install.DefaultPluginInstallationService] execute com.atlassian.upm.spi.PluginInstallException: Unknown error, plugin not installed
 -- referer: http://localhost:8561/c561/plugins/servlet/upm | url: /c561/rest/plugins/1.0/ | userName: admin
com.atlassian.upm.spi.PluginInstallException: Unknown error, plugin not installed
	at com.atlassian.upm.core.install.ObrPluginInstallHandler.installResources(ObrPluginInstallHandler.java:228)
	at com.atlassian.upm.core.install.ObrPluginInstallHandler.installPluginInternal(ObrPluginInstallHandler.java:146)
	at com.atlassian.upm.core.install.AbstractPluginInstallHandler.installPlugin(AbstractPluginInstallHandler.java:48)
	at com.atlassian.upm.core.install.DefaultPluginInstallationService.execute(DefaultPluginInstallationService.java:111)
	at com.atlassian.upm.core.install.DefaultPluginInstallationService.install(DefaultPluginInstallationService.java:87)
	at com.atlassian.upm.install.UpmPluginInstallationService.install(UpmPluginInstallationService.java:94)
	at com.atlassian.upm.core.rest.resources.install.InstallTask.installFromFile(InstallTask.java:123)
	at com.atlassian.upm.core.rest.resources.install.InstallFromFileTask.executeTask(InstallFromFileTask.java:38)
	at com.atlassian.upm.core.rest.resources.install.InstallTask.call(InstallTask.java:60)
	at com.atlassian.upm.core.rest.resources.install.InstallTask.call(InstallTask.java:34)
	at com.atlassian.upm.core.rest.async.AsynchronousTaskManager$1.call(AsynchronousTaskManager.java:78)
	at com.atlassian.upm.core.rest.async.AsynchronousTaskManager$1.call(AsynchronousTaskManager.java:73)
	at com.atlassian.sal.core.executor.ThreadLocalDelegateCallable.call(ThreadLocalDelegateCallable.java:42)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
	at java.util.concurrent.FutureTask.run(FutureTask.java:166)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
	at java.lang.Thread.run(Thread.java:722)