The XA-datasource configuration is not so immediate (for me it wasn't ). So please consider this datasource in case you use postgres.

<xa-datasource>
    <jndi-name>open18JbpmDatasource</jndi-name>
    <use-java-context>false</use-java-context>
    <xa-datasource-class>org.postgresql.xa.PGXADataSource</xa-datasource-class>
    <xa-datasource-property name="ServerName">localhost</xa-datasource-property>
    <xa-datasource-property name="PortNumber">5432</xa-datasource-property>
    <xa-datasource-property name="DatabaseName">jbpmOpen18</xa-datasource-property>
    <xa-datasource-property name="User">jbpm</xa-datasource-property>
    <xa-datasource-property name="Password">jbpm</xa-datasource-property>
    <track-connection-by-tx />
</xa-datasource>