[JBoss] No ManagedConnections available / JCA max connection (session)

It seems that with Jackrabbit JCA 2.0.0 deployed on JBoss 4.2.3.GA, "only" 20 session can be opened at the same time.
When trying to open the 21th session, I get the following exception :

15:49:16,047 ERROR [STDERR] Caused by: javax.resource.ResourceException: No ManagedConnections available within configured blocking timeout ( 30000 [ms] )
15:49:16,047 ERROR [STDERR] at org.jboss.resource.connectionmanager.InternalManagedConnectionPool.getConnection(InternalManagedConnectionPool.java:303)
15:49:16,047 ERROR [STDERR] at org.jboss.resource.connectionmanager.JBossManagedConnectionPool$BasePool.getConnection(JBossManagedConnectionPool.java:575)
15:49:16,047 ERROR [STDERR] at org.jboss.resource.connectionmanager.BaseConnectionManager2.getManagedConnection(BaseConnectionManager2.java:347)
15:49:16,047 ERROR [STDERR] at org.jboss.resource.connectionmanager.TxConnectionManager.getManagedConnection(TxConnectionManager.java:330)
15:49:16,047 ERROR [STDERR] at org.jboss.resource.connectionmanager.BaseConnectionManager2.allocateConnection(BaseConnectionManager2.java:402)
15:49:16,047 ERROR [STDERR] at org.jboss.resource.connectionmanager.BaseConnectionManager2$ConnectionManagerProxy.allocateConnection(BaseConnectionManager2.java:849)
15:49:16,047 ERROR [STDERR] at org.apache.jackrabbit.jca.JCARepositoryHandle.login(JCARepositoryHandle.java:72)
15:49:16,047 ERROR [STDERR] ... 28 more

I don't see this limit / max count in Jackrabbit JCA configuration. I've checked whether it comes from underlying PersistenceManager.
Persistence manager is postgresql bundle one from JNDI (with javax.naming.InitialContext as driver).
But JBoss JMX console displayed it still have available connection to used database whereas Jackrabbit JCA keep throwing exception.
Moreover I'm still able to use underlying datasource to get a JDBC connection and execute a PreparedStatement ("SELECT * FROM default_bundle" as test).

So as I think underlying datasource/persistence manager is ok, there may be a trouble with Jackrabbit JCA.