Datasource dbcp.numActive not found

I just installed jira 6.4.5 in my environment connecting to an Oracle database.
I can configure it and it seems all fine (can create projects and issues) but I have right from the beginning the following message in the log:
2015-09-28 16:23:41,360 jira-monitoring-plugin ERROR [jira.plugins.monitor.MetricsCollectorTask] Error running task
java.lang.IllegalArgumentException: Datasource dbcp.numActive not found
at org.rrd4j.core.Sample.setValue(Sample.java:63)
at com.atlassian.jira.plugins.monitor.database.ConnectionPoolGraph.setDatasourceValues(ConnectionPoolGraph.java:45)
at com.atlassian.jira.plugins.monitor.rrd4j.RrdUpdater.addSample(RrdUpdater.java:76)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.doRun(MetricsCollectorTask.java:52)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.run(MetricsCollectorTask.java:41)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)
2015-09-28 16:23:51,360 jira-monitoring-plugin ERROR [jira.plugins.monitor.MetricsCollectorTask] Error running task
java.lang.IllegalArgumentException: Datasource dbcp.numActive not found
at org.rrd4j.core.Sample.setValue(Sample.java:63)
at com.atlassian.jira.plugins.monitor.database.ConnectionPoolGraph.setDatasourceValues(ConnectionPoolGraph.java:45)
at com.atlassian.jira.plugins.monitor.rrd4j.RrdUpdater.addSample(RrdUpdater.java:76)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.doRun(MetricsCollectorTask.java:52)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.run(MetricsCollectorTask.java:41)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)
2015-09-28 16:24:01,360 jira-monitoring-plugin ERROR [jira.plugins.monitor.MetricsCollectorTask] Error running task
java.lang.IllegalArgumentException: Datasource dbcp.numActive not found
at org.rrd4j.core.Sample.setValue(Sample.java:63)
at com.atlassian.jira.plugins.monitor.database.ConnectionPoolGraph.setDatasourceValues(ConnectionPoolGraph.java:45)
at com.atlassian.jira.plugins.monitor.rrd4j.RrdUpdater.addSample(RrdUpdater.java:76)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.doRun(MetricsCollectorTask.java:52)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.run(MetricsCollectorTask.java:41)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)

an entry is added every 10 seconds in the log.
And if I try to access the Database monitoring in the Add-ons tab I get a full screen of errors, starting with:
2015-09-28 16:23:41,360 jira-monitoring-plugin ERROR [jira.plugins.monitor.MetricsCollectorTask] Error running task
java.lang.IllegalArgumentException: Datasource dbcp.numActive not found
at org.rrd4j.core.Sample.setValue(Sample.java:63)
at com.atlassian.jira.plugins.monitor.database.ConnectionPoolGraph.setDatasourceValues(ConnectionPoolGraph.java:45)
at com.atlassian.jira.plugins.monitor.rrd4j.RrdUpdater.addSample(RrdUpdater.java:76)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.doRun(MetricsCollectorTask.java:52)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.run(MetricsCollectorTask.java:41)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)
2015-09-28 16:23:51,360 jira-monitoring-plugin ERROR [jira.plugins.monitor.MetricsCollectorTask] Error running task
java.lang.IllegalArgumentException: Datasource dbcp.numActive not found
at org.rrd4j.core.Sample.setValue(Sample.java:63)
at com.atlassian.jira.plugins.monitor.database.ConnectionPoolGraph.setDatasourceValues(ConnectionPoolGraph.java:45)
at com.atlassian.jira.plugins.monitor.rrd4j.RrdUpdater.addSample(RrdUpdater.java:76)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.doRun(MetricsCollectorTask.java:52)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.run(MetricsCollectorTask.java:41)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)
2015-09-28 16:24:01,360 jira-monitoring-plugin ERROR [jira.plugins.monitor.MetricsCollectorTask] Error running task
java.lang.IllegalArgumentException: Datasource dbcp.numActive not found
at org.rrd4j.core.Sample.setValue(Sample.java:63)
at com.atlassian.jira.plugins.monitor.database.ConnectionPoolGraph.setDatasourceValues(ConnectionPoolGraph.java:45)
at com.atlassian.jira.plugins.monitor.rrd4j.RrdUpdater.addSample(RrdUpdater.java:76)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.doRun(MetricsCollectorTask.java:52)
at com.atlassian.jira.plugins.monitor.MetricsCollectorTask.run(MetricsCollectorTask.java:41)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)

Obviously there is something wrong in my environment, but what?

Thanks to have a look into that.
S. Ruffin